diff --git a/markitect/finance/cli.py b/markitect/finance/cli.py
index 202f2d3c..e623c0be 100644
--- a/markitect/finance/cli.py
+++ b/markitect/finance/cli.py
@@ -7,7 +7,7 @@ including report generation, cost item management, and period calculations.
 
 import click
 import sys
-from datetime import date, datetime
+from datetime import date, datetime, timedelta
 from decimal import Decimal
 from pathlib import Path
 from typing import Optional
@@ -15,6 +15,7 @@ from typing import Optional
 from .cost_manager import CostItemManager, CostItem
 from .report_generator import CostReportGenerator, ReportConfig
 from .session_tracker import SessionCostTracker
+from .period_manager import PeriodManager, PeriodStatus
 from ..config_manager import ConfigurationManager
 
 
@@ -546,4 +547,365 @@ def session_summary(issue_ids: Optional[str], db_path: Optional[str]):
 
     except Exception as e:
         click.echo(f"Error getting session summary: {e}", err=True)
+        sys.exit(1)
+
+
+@cost_commands.group(name='period')
+def cost_period():
+    """Manage calculation periods and lifecycle."""
+    pass
+
+
+@cost_period.command('create')
+@click.option('--start-date', required=True, help='Period start date (YYYY-MM-DD)')
+@click.option('--end-date', required=True, help='Period end date (YYYY-MM-DD)')
+@click.option('--type', 'period_type', default='monthly', help='Period type (monthly, quarterly, yearly)')
+@click.option('--loss-forward', type=float, help='Loss carried forward from previous period')
+@click.option('--database', 'db_path', help='Database path (defaults to config)')
+def create_period(start_date: str, end_date: str, period_type: str,
+                 loss_forward: Optional[float], db_path: Optional[str]):
+    """Create a new calculation period."""
+    try:
+        # Get database path
+        if not db_path:
+            config_manager = ConfigurationManager()
+            config = config_manager.get_current_config()
+            db_path = config.get('database_path')
+
+        if not db_path:
+            click.echo("Error: No database path specified.", err=True)
+            sys.exit(1)
+
+        # Parse dates
+        try:
+            start_date_obj = datetime.strptime(start_date, '%Y-%m-%d').date()
+            end_date_obj = datetime.strptime(end_date, '%Y-%m-%d').date()
+        except ValueError:
+            click.echo("Error: Dates must be in YYYY-MM-DD format", err=True)
+            sys.exit(1)
+
+        # Initialize period manager
+        period_manager = PeriodManager(db_path)
+
+        # Convert loss forward to Decimal
+        loss_forward_decimal = None
+        if loss_forward is not None:
+            loss_forward_decimal = Decimal(str(loss_forward))
+
+        # Create period
+        period_id = period_manager.create_period(
+            period_start=start_date_obj,
+            period_end=end_date_obj,
+            period_type=period_type,
+            loss_carried_forward=loss_forward_decimal
+        )
+
+        if period_id:
+            click.echo(f"✅ Created period #{period_id}")
+            click.echo(f"📅 Period: {start_date} to {end_date}")
+            click.echo(f"📊 Type: {period_type}")
+
+            if loss_forward:
+                click.echo(f"💸 Loss carried forward: €{loss_forward:.4f}")
+        else:
+            click.echo("❌ Failed to create period", err=True)
+            sys.exit(1)
+
+    except ValueError as e:
+        click.echo(f"Error: {e}", err=True)
+        sys.exit(1)
+    except Exception as e:
+        click.echo(f"Error creating period: {e}", err=True)
+        sys.exit(1)
+
+
+@cost_period.command('list')
+@click.option('--status', type=click.Choice(['open', 'calculating', 'closed']),
+              help='Filter by status')
+@click.option('--start-from', help='Show periods starting from date (YYYY-MM-DD)')
+@click.option('--end-before', help='Show periods ending before date (YYYY-MM-DD)')
+@click.option('--database', 'db_path', help='Database path (defaults to config)')
+def list_periods(status: Optional[str], start_from: Optional[str],
+                end_before: Optional[str], db_path: Optional[str]):
+    """List calculation periods with optional filtering."""
+    try:
+        # Get database path
+        if not db_path:
+            config_manager = ConfigurationManager()
+            config = config_manager.get_current_config()
+            db_path = config.get('database_path')
+
+        if not db_path:
+            click.echo("Error: No database path specified.", err=True)
+            sys.exit(1)
+
+        # Parse date filters
+        start_date_obj = None
+        end_date_obj = None
+
+        if start_from:
+            try:
+                start_date_obj = datetime.strptime(start_from, '%Y-%m-%d').date()
+            except ValueError:
+                click.echo("Error: Start date must be in YYYY-MM-DD format", err=True)
+                sys.exit(1)
+
+        if end_before:
+            try:
+                end_date_obj = datetime.strptime(end_before, '%Y-%m-%d').date()
+            except ValueError:
+                click.echo("Error: End date must be in YYYY-MM-DD format", err=True)
+                sys.exit(1)
+
+        # Get periods
+        period_manager = PeriodManager(db_path)
+        periods = period_manager.list_periods(
+            status_filter=status,
+            start_date=start_date_obj,
+            end_date=end_date_obj
+        )
+
+        if not periods:
+            click.echo("No periods found matching criteria.")
+            return
+
+        # Display periods
+        click.echo(f"📅 Calculation Periods")
+        click.echo("=" * 80)
+        click.echo(f"{'ID':<4} {'Start Date':<12} {'End Date':<12} {'Type':<10} {'Status':<12} {'Total Cost':<12}")
+        click.echo("-" * 80)
+
+        for period in periods:
+            click.echo(
+                f"{period['id']:<4} {period['period_start']:<12} {period['period_end']:<12} "
+                f"{period['period_type']:<10} {period['status']:<12} €{float(period['total_costs']):<11.2f}"
+            )
+
+        click.echo(f"\nTotal: {len(periods)} periods")
+
+    except Exception as e:
+        click.echo(f"Error listing periods: {e}", err=True)
+        sys.exit(1)
+
+
+@cost_period.command('show')
+@click.argument('period_id', type=int)
+@click.option('--database', 'db_path', help='Database path (defaults to config)')
+def show_period(period_id: int, db_path: Optional[str]):
+    """Show detailed information for a specific period."""
+    try:
+        # Get database path
+        if not db_path:
+            config_manager = ConfigurationManager()
+            config = config_manager.get_current_config()
+            db_path = config.get('database_path')
+
+        if not db_path:
+            click.echo("Error: No database path specified.", err=True)
+            sys.exit(1)
+
+        # Get period details
+        period_manager = PeriodManager(db_path)
+        period = period_manager.get_period_by_id(period_id)
+
+        if not period:
+            click.echo(f"Period #{period_id} not found.", err=True)
+            sys.exit(1)
+
+        # Display period details
+        click.echo(f"📅 Period #{period['id']} Details")
+        click.echo("=" * 40)
+        click.echo(f"Start Date: {period['period_start']}")
+        click.echo(f"End Date: {period['period_end']}")
+        click.echo(f"Type: {period['period_type']}")
+        click.echo(f"Status: {period['status']}")
+        click.echo(f"Total Costs: €{float(period['total_costs']):.4f}")
+        click.echo(f"Active Issues: {period['active_issues_count']}")
+
+        if period['cost_per_issue'] > 0:
+            click.echo(f"Cost per Issue: €{float(period['cost_per_issue']):.4f}")
+
+        if period['loss_carried_forward'] > 0:
+            click.echo(f"Loss Carried Forward: €{float(period['loss_carried_forward']):.4f}")
+
+        click.echo(f"Created: {period['created_at']}")
+
+        if period['updated_at']:
+            click.echo(f"Updated: {period['updated_at']}")
+
+    except Exception as e:
+        click.echo(f"Error showing period: {e}", err=True)
+        sys.exit(1)
+
+
+@cost_period.command('calculate')
+@click.argument('period_id', type=int)
+@click.option('--database', 'db_path', help='Database path (defaults to config)')
+def calculate_period(period_id: int, db_path: Optional[str]):
+    """Calculate costs for a specific period."""
+    try:
+        # Get database path
+        if not db_path:
+            config_manager = ConfigurationManager()
+            config = config_manager.get_current_config()
+            db_path = config.get('database_path')
+
+        if not db_path:
+            click.echo("Error: No database path specified.", err=True)
+            sys.exit(1)
+
+        # Calculate period costs
+        period_manager = PeriodManager(db_path)
+        calculation = period_manager.calculate_period_costs(period_id)
+
+        click.echo(f"📊 Period #{calculation['period_id']} Cost Calculation")
+        click.echo("=" * 50)
+        click.echo(f"Period: {calculation['period_start']} to {calculation['period_end']}")
+        click.echo(f"Monthly Recurring: €{calculation['monthly_costs']:.2f}")
+        click.echo(f"One-time Expenses: €{calculation['one_time_costs']:.2f}")
+
+        if calculation['loss_carried_forward'] > 0:
+            click.echo(f"Loss Carried Forward: €{calculation['loss_carried_forward']:.2f}")
+
+        click.echo(f"Total Period Cost: €{calculation['total_costs']:.2f}")
+        click.echo(f"Calculated: {calculation['calculation_date'][:19]}")
+
+    except Exception as e:
+        click.echo(f"Error calculating period: {e}", err=True)
+        sys.exit(1)
+
+
+@cost_period.command('status')
+@click.argument('period_id', type=int)
+@click.argument('new_status', type=click.Choice(['open', 'calculating', 'closed']))
+@click.option('--database', 'db_path', help='Database path (defaults to config)')
+def update_status(period_id: int, new_status: str, db_path: Optional[str]):
+    """Update period status."""
+    try:
+        # Get database path
+        if not db_path:
+            config_manager = ConfigurationManager()
+            config = config_manager.get_current_config()
+            db_path = config.get('database_path')
+
+        if not db_path:
+            click.echo("Error: No database path specified.", err=True)
+            sys.exit(1)
+
+        # Update status
+        period_manager = PeriodManager(db_path)
+        success = period_manager.update_period_status(period_id, new_status)
+
+        if success:
+            click.echo(f"✅ Period #{period_id} status updated to '{new_status}'")
+        else:
+            click.echo(f"❌ Failed to update period #{period_id} status", err=True)
+            sys.exit(1)
+
+    except ValueError as e:
+        click.echo(f"Error: {e}", err=True)
+        sys.exit(1)
+    except Exception as e:
+        click.echo(f"Error updating status: {e}", err=True)
+        sys.exit(1)
+
+
+@cost_period.command('close')
+@click.argument('period_id', type=int)
+@click.option('--skip-validation', is_flag=True, help='Skip transaction validation')
+@click.option('--database', 'db_path', help='Database path (defaults to config)')
+def close_period(period_id: int, skip_validation: bool, db_path: Optional[str]):
+    """Close a period after final calculations."""
+    try:
+        # Get database path
+        if not db_path:
+            config_manager = ConfigurationManager()
+            config = config_manager.get_current_config()
+            db_path = config.get('database_path')
+
+        if not db_path:
+            click.echo("Error: No database path specified.", err=True)
+            sys.exit(1)
+
+        # Close period
+        period_manager = PeriodManager(db_path)
+        success = period_manager.close_period(
+            period_id,
+            validate_transactions=not skip_validation
+        )
+
+        if success:
+            click.echo(f"✅ Period #{period_id} has been closed")
+
+            # Show final calculation
+            period = period_manager.get_period_by_id(period_id)
+            if period:
+                click.echo(f"💰 Final total cost: €{float(period['total_costs']):.4f}")
+        else:
+            click.echo(f"❌ Failed to close period #{period_id}", err=True)
+            sys.exit(1)
+
+    except ValueError as e:
+        click.echo(f"Error: {e}", err=True)
+        sys.exit(1)
+    except Exception as e:
+        click.echo(f"Error closing period: {e}", err=True)
+        sys.exit(1)
+
+
+@cost_period.command('current')
+@click.option('--date', 'date_str', help='Date to find period for (YYYY-MM-DD, defaults to today)')
+@click.option('--database', 'db_path', help='Database path (defaults to config)')
+def current_period(date_str: Optional[str], db_path: Optional[str]):
+    """Show current active period for a given date."""
+    try:
+        # Get database path
+        if not db_path:
+            config_manager = ConfigurationManager()
+            config = config_manager.get_current_config()
+            db_path = config.get('database_path')
+
+        if not db_path:
+            click.echo("Error: No database path specified.", err=True)
+            sys.exit(1)
+
+        # Parse date if provided
+        target_date = None
+        if date_str:
+            try:
+                target_date = datetime.strptime(date_str, '%Y-%m-%d').date()
+            except ValueError:
+                click.echo("Error: Date must be in YYYY-MM-DD format", err=True)
+                sys.exit(1)
+
+        # Get current period
+        period_manager = PeriodManager(db_path)
+        current = period_manager.get_current_period(target_date)
+
+        if not current:
+            date_display = date_str if date_str else "today"
+            click.echo(f"No active period found for {date_display}")
+
+            # Suggest creating a period
+            if not date_str:
+                today = date.today()
+                # Calculate last day of current month
+                if today.month == 12:
+                    next_month = today.replace(year=today.year + 1, month=1, day=1)
+                else:
+                    next_month = today.replace(month=today.month + 1, day=1)
+                last_day = (next_month - timedelta(days=1)).day
+                click.echo(f"💡 Create one with: markitect cost period create --start-date {today.year}-{today.month:02d}-01 --end-date {today.year}-{today.month:02d}-{last_day:02d}")
+            return
+
+        # Display current period
+        click.echo(f"📅 Current Active Period")
+        click.echo("=" * 30)
+        click.echo(f"Period #{current['id']}")
+        click.echo(f"Dates: {current['period_start']} to {current['period_end']}")
+        click.echo(f"Status: {current['status']}")
+        click.echo(f"Total Costs: €{float(current['total_costs']):.4f}")
+
+    except Exception as e:
+        click.echo(f"Error getting current period: {e}", err=True)
         sys.exit(1)
\ No newline at end of file
diff --git a/markitect/finance/period_manager.py b/markitect/finance/period_manager.py
new file mode 100644
index 00000000..6e083d67
--- /dev/null
+++ b/markitect/finance/period_manager.py
@@ -0,0 +1,568 @@
+"""
+Period Management Framework for MarkiTect Cost Tracking System.
+
+This module provides comprehensive period lifecycle management including:
+- Period creation, status management, and transitions
+- Period overlap validation and conflict resolution
+- Automatic period calculations and data aggregation
+- Loss carried forward calculations
+- Period closure validation and audit trails
+
+The framework supports the complete period lifecycle:
+1. OPEN: Period is created and active for cost tracking
+2. CALCULATING: Period is being processed for cost allocation
+3. CLOSED: Period is finalized with all calculations complete
+"""
+
+import sqlite3
+import os
+from datetime import datetime, date, timedelta
+from decimal import Decimal
+from typing import Optional, Dict, Any, List, Tuple
+from dataclasses import dataclass
+from enum import Enum
+
+from .models import FinanceModels
+
+
+class PeriodStatus(Enum):
+    """Period status enumeration."""
+    OPEN = 'open'
+    CALCULATING = 'calculating'
+    CLOSED = 'closed'
+
+
+@dataclass
+class Period:
+    """Period data model."""
+    id: Optional[int] = None
+    period_start: Optional[date] = None
+    period_end: Optional[date] = None
+    period_type: str = 'monthly'
+    status: str = 'open'
+    total_costs: Decimal = Decimal('0.00')
+    active_issues_count: int = 0
+    cost_per_issue: Decimal = Decimal('0.00')
+    loss_carried_forward: Decimal = Decimal('0.00')
+    created_at: Optional[datetime] = None
+    updated_at: Optional[datetime] = None
+
+
+class PeriodManager:
+    """
+    Comprehensive period management system for cost tracking.
+
+    Handles period lifecycle management, status transitions, calculations,
+    and validation for the cost tracking system.
+    """
+
+    def __init__(self, db_path: str):
+        """
+        Initialize period manager.
+
+        Args:
+            db_path: Path to SQLite database file
+        """
+        self.db_path = db_path
+        self.finance_models = FinanceModels(db_path)
+
+    def create_period(self, period_start: date, period_end: date,
+                     period_type: str = 'monthly',
+                     loss_carried_forward: Optional[Decimal] = None) -> Optional[int]:
+        """
+        Create a new calculation period with validation.
+
+        Args:
+            period_start: Start date of the period
+            period_end: End date of the period
+            period_type: Type of period (monthly, quarterly, yearly)
+            loss_carried_forward: Amount carried forward from previous period
+
+        Returns:
+            ID of created period, or None if creation failed
+
+        Raises:
+            ValueError: If period dates are invalid or overlapping periods exist
+        """
+        # Validate period dates
+        if period_end <= period_start:
+            raise ValueError("Period end date must be after start date")
+
+        # Check for overlapping periods
+        overlapping = self.find_overlapping_periods(period_start, period_end)
+        if overlapping:
+            overlapping_periods = [f"#{p['id']} ({p['period_start']} to {p['period_end']})"
+                                 for p in overlapping]
+            raise ValueError(f"Period overlaps with existing periods: {', '.join(overlapping_periods)}")
+
+        # Set loss carried forward to 0 if not provided
+        if loss_carried_forward is None:
+            loss_carried_forward = Decimal('0.00')
+
+        conn = self.finance_models.get_connection()
+        cursor = conn.cursor()
+
+        try:
+            cursor.execute('''
+                INSERT INTO cost_periods (
+                    period_start, period_end, period_type, status,
+                    total_costs, active_issues_count, cost_per_issue,
+                    loss_carried_forward
+                ) VALUES (?, ?, ?, ?, ?, ?, ?, ?)
+            ''', (
+                period_start.isoformat(),
+                period_end.isoformat(),
+                period_type,
+                PeriodStatus.OPEN.value,
+                0,
+                0,
+                0,
+                float(loss_carried_forward)
+            ))
+
+            conn.commit()
+            period_id = cursor.lastrowid
+            return period_id
+
+        except sqlite3.Error as e:
+            conn.rollback()
+            raise RuntimeError(f"Failed to create period: {e}")
+
+        finally:
+            conn.close()
+
+    def get_period_by_id(self, period_id: int) -> Optional[Dict[str, Any]]:
+        """
+        Get period by ID.
+
+        Args:
+            period_id: Period ID
+
+        Returns:
+            Period dictionary or None if not found
+        """
+        conn = self.finance_models.get_connection()
+        cursor = conn.cursor()
+
+        try:
+            cursor.execute('''
+                SELECT id, period_start, period_end, period_type, status,
+                       total_costs, active_issues_count, cost_per_issue,
+                       loss_carried_forward, created_at, updated_at
+                FROM cost_periods
+                WHERE id = ?
+            ''', (period_id,))
+
+            row = cursor.fetchone()
+            if not row:
+                return None
+
+            return {
+                'id': row[0],
+                'period_start': row[1],
+                'period_end': row[2],
+                'period_type': row[3],
+                'status': row[4],
+                'total_costs': Decimal(str(row[5])),
+                'active_issues_count': row[6],
+                'cost_per_issue': Decimal(str(row[7])),
+                'loss_carried_forward': Decimal(str(row[8])),
+                'created_at': row[9],
+                'updated_at': row[10]
+            }
+
+        except sqlite3.Error as e:
+            raise RuntimeError(f"Failed to get period: {e}")
+
+        finally:
+            conn.close()
+
+    def find_overlapping_periods(self, period_start: date, period_end: date,
+                                exclude_period_id: Optional[int] = None) -> List[Dict[str, Any]]:
+        """
+        Find periods that overlap with the given date range.
+
+        Args:
+            period_start: Start date to check
+            period_end: End date to check
+            exclude_period_id: Period ID to exclude from check (for updates)
+
+        Returns:
+            List of overlapping periods
+        """
+        conn = self.finance_models.get_connection()
+        cursor = conn.cursor()
+
+        try:
+            sql = '''
+                SELECT id, period_start, period_end, status
+                FROM cost_periods
+                WHERE NOT (period_end < ? OR period_start > ?)
+            '''
+            params = [period_start.isoformat(), period_end.isoformat()]
+
+            if exclude_period_id:
+                sql += ' AND id != ?'
+                params.append(exclude_period_id)
+
+            cursor.execute(sql, params)
+            rows = cursor.fetchall()
+
+            return [
+                {
+                    'id': row[0],
+                    'period_start': row[1],
+                    'period_end': row[2],
+                    'status': row[3]
+                }
+                for row in rows
+            ]
+
+        except sqlite3.Error as e:
+            raise RuntimeError(f"Failed to check overlapping periods: {e}")
+
+        finally:
+            conn.close()
+
+    def update_period_status(self, period_id: int, new_status: str) -> bool:
+        """
+        Update period status with validation.
+
+        Args:
+            period_id: Period ID to update
+            new_status: New status (open, calculating, closed)
+
+        Returns:
+            True if update successful, False otherwise
+
+        Raises:
+            ValueError: If status transition is invalid
+        """
+        valid_statuses = [status.value for status in PeriodStatus]
+        if new_status not in valid_statuses:
+            raise ValueError(f"Invalid status '{new_status}'. Valid statuses: {valid_statuses}")
+
+        # Get current period to validate status transition
+        current_period = self.get_period_by_id(period_id)
+        if not current_period:
+            raise ValueError(f"Period #{period_id} not found")
+
+        current_status = current_period['status']
+
+        # Validate status transitions
+        valid_transitions = {
+            PeriodStatus.OPEN.value: [PeriodStatus.CALCULATING.value, PeriodStatus.CLOSED.value],
+            PeriodStatus.CALCULATING.value: [PeriodStatus.OPEN.value, PeriodStatus.CLOSED.value],
+            PeriodStatus.CLOSED.value: [PeriodStatus.OPEN.value]  # Allow reopening if needed
+        }
+
+        if new_status not in valid_transitions.get(current_status, []):
+            raise ValueError(f"Invalid status transition from '{current_status}' to '{new_status}'")
+
+        conn = self.finance_models.get_connection()
+        cursor = conn.cursor()
+
+        try:
+            cursor.execute('''
+                UPDATE cost_periods
+                SET status = ?, updated_at = CURRENT_TIMESTAMP
+                WHERE id = ?
+            ''', (new_status, period_id))
+
+            conn.commit()
+            return cursor.rowcount > 0
+
+        except sqlite3.Error as e:
+            conn.rollback()
+            raise RuntimeError(f"Failed to update period status: {e}")
+
+        finally:
+            conn.close()
+
+    def calculate_period_costs(self, period_id: int) -> Dict[str, Any]:
+        """
+        Calculate total costs for a period based on active cost items.
+
+        Args:
+            period_id: Period ID to calculate
+
+        Returns:
+            Dictionary with calculation results
+        """
+        period = self.get_period_by_id(period_id)
+        if not period:
+            raise ValueError(f"Period #{period_id} not found")
+
+        period_start = datetime.strptime(period['period_start'], '%Y-%m-%d').date()
+        period_end = datetime.strptime(period['period_end'], '%Y-%m-%d').date()
+
+        # Import CostItemManager to get cost data
+        from .cost_manager import CostItemManager
+        cost_manager = CostItemManager(self.db_path)
+
+        # Get all active cost items
+        cost_items = cost_manager.list_cost_items(active_only=True)
+
+        total_monthly = Decimal('0')
+        total_one_time = Decimal('0')
+
+        for item in cost_items:
+            item_start = datetime.strptime(item['starting_from_date'], '%Y-%m-%d').date()
+
+            # Skip items that start after our period
+            if item_start > period_end:
+                continue
+
+            # Handle ending date
+            item_end = None
+            if item['ending_date']:
+                item_end = datetime.strptime(item['ending_date'], '%Y-%m-%d').date()
+                # Skip items that ended before our period
+                if item_end < period_start:
+                    continue
+
+            # Calculate cost for this period
+            if item['cost_type'] == 'monthly':
+                # For monthly items, include full amount if active during period
+                total_monthly += Decimal(str(item['amount_eur']))
+            elif item['cost_type'] == 'one_time':
+                # For one-time items, include only if starting date is within period
+                if period_start <= item_start <= period_end:
+                    total_one_time += Decimal(str(item['amount_eur']))
+
+        total_costs = total_monthly + total_one_time + period['loss_carried_forward']
+
+        # Update period with calculated values
+        conn = self.finance_models.get_connection()
+        cursor = conn.cursor()
+
+        try:
+            cursor.execute('''
+                UPDATE cost_periods
+                SET total_costs = ?, updated_at = CURRENT_TIMESTAMP
+                WHERE id = ?
+            ''', (float(total_costs), period_id))
+
+            conn.commit()
+
+        except sqlite3.Error as e:
+            conn.rollback()
+            raise RuntimeError(f"Failed to update period calculations: {e}")
+
+        finally:
+            conn.close()
+
+        return {
+            'period_id': period_id,
+            'period_start': period['period_start'],
+            'period_end': period['period_end'],
+            'monthly_costs': float(total_monthly),
+            'one_time_costs': float(total_one_time),
+            'loss_carried_forward': float(period['loss_carried_forward']),
+            'total_costs': float(total_costs),
+            'calculation_date': datetime.now().isoformat()
+        }
+
+    def close_period(self, period_id: int, validate_transactions: bool = True) -> bool:
+        """
+        Close a period after validation.
+
+        Args:
+            period_id: Period ID to close
+            validate_transactions: Whether to validate all transactions are recorded
+
+        Returns:
+            True if period closed successfully, False otherwise
+
+        Raises:
+            ValueError: If period cannot be closed due to validation errors
+        """
+        period = self.get_period_by_id(period_id)
+        if not period:
+            raise ValueError(f"Period #{period_id} not found")
+
+        if period['status'] == PeriodStatus.CLOSED.value:
+            return True  # Already closed
+
+        # Optional transaction validation
+        if validate_transactions:
+            # Check if there are any unrecorded transactions
+            # This is a placeholder for future transaction validation logic
+            pass
+
+        # Calculate final costs before closing
+        calculation_result = self.calculate_period_costs(period_id)
+
+        # Update status to closed
+        return self.update_period_status(period_id, PeriodStatus.CLOSED.value)
+
+    def list_periods(self, status_filter: Optional[str] = None,
+                    start_date: Optional[date] = None,
+                    end_date: Optional[date] = None) -> List[Dict[str, Any]]:
+        """
+        List periods with optional filtering.
+
+        Args:
+            status_filter: Filter by status (open, calculating, closed)
+            start_date: Filter periods starting from this date
+            end_date: Filter periods ending before this date
+
+        Returns:
+            List of periods matching criteria
+        """
+        conn = self.finance_models.get_connection()
+        cursor = conn.cursor()
+
+        try:
+            sql = '''
+                SELECT id, period_start, period_end, period_type, status,
+                       total_costs, active_issues_count, cost_per_issue,
+                       loss_carried_forward, created_at, updated_at
+                FROM cost_periods
+                WHERE 1=1
+            '''
+            params = []
+
+            if status_filter:
+                sql += ' AND status = ?'
+                params.append(status_filter)
+
+            if start_date:
+                sql += ' AND period_start >= ?'
+                params.append(start_date.isoformat())
+
+            if end_date:
+                sql += ' AND period_end <= ?'
+                params.append(end_date.isoformat())
+
+            sql += ' ORDER BY period_start DESC'
+
+            cursor.execute(sql, params)
+            rows = cursor.fetchall()
+
+            return [
+                {
+                    'id': row[0],
+                    'period_start': row[1],
+                    'period_end': row[2],
+                    'period_type': row[3],
+                    'status': row[4],
+                    'total_costs': Decimal(str(row[5])),
+                    'active_issues_count': row[6],
+                    'cost_per_issue': Decimal(str(row[7])),
+                    'loss_carried_forward': Decimal(str(row[8])),
+                    'created_at': row[9],
+                    'updated_at': row[10]
+                }
+                for row in rows
+            ]
+
+        except sqlite3.Error as e:
+            raise RuntimeError(f"Failed to list periods: {e}")
+
+        finally:
+            conn.close()
+
+    def get_current_period(self, target_date: Optional[date] = None) -> Optional[Dict[str, Any]]:
+        """
+        Get the current active period for a given date.
+
+        Args:
+            target_date: Date to find period for (defaults to today)
+
+        Returns:
+            Current period dictionary or None if no period found
+        """
+        if target_date is None:
+            target_date = date.today()
+
+        conn = self.finance_models.get_connection()
+        cursor = conn.cursor()
+
+        try:
+            cursor.execute('''
+                SELECT id, period_start, period_end, period_type, status,
+                       total_costs, active_issues_count, cost_per_issue,
+                       loss_carried_forward, created_at, updated_at
+                FROM cost_periods
+                WHERE period_start <= ? AND period_end >= ?
+                AND status IN ('open', 'calculating')
+                ORDER BY period_start DESC
+                LIMIT 1
+            ''', (target_date.isoformat(), target_date.isoformat()))
+
+            row = cursor.fetchone()
+            if not row:
+                return None
+
+            return {
+                'id': row[0],
+                'period_start': row[1],
+                'period_end': row[2],
+                'period_type': row[3],
+                'status': row[4],
+                'total_costs': Decimal(str(row[5])),
+                'active_issues_count': row[6],
+                'cost_per_issue': Decimal(str(row[7])),
+                'loss_carried_forward': Decimal(str(row[8])),
+                'created_at': row[9],
+                'updated_at': row[10]
+            }
+
+        except sqlite3.Error as e:
+            raise RuntimeError(f"Failed to get current period: {e}")
+
+        finally:
+            conn.close()
+
+    def create_monthly_period(self, year: int, month: int) -> Optional[int]:
+        """
+        Convenience method to create a monthly period.
+
+        Args:
+            year: Year of the period
+            month: Month of the period (1-12)
+
+        Returns:
+            ID of created period, or None if creation failed
+        """
+        # Calculate period dates
+        period_start = date(year, month, 1)
+
+        # Calculate last day of month
+        if month == 12:
+            next_month_start = date(year + 1, 1, 1)
+        else:
+            next_month_start = date(year, month + 1, 1)
+
+        period_end = next_month_start - timedelta(days=1)
+
+        return self.create_period(period_start, period_end, 'monthly')
+
+    def auto_create_period_for_date(self, target_date: date) -> Optional[int]:
+        """
+        Automatically create a monthly period for a given date if it doesn't exist.
+
+        Args:
+            target_date: Date that should be covered by a period
+
+        Returns:
+            ID of existing or newly created period, or None if creation failed
+        """
+        # Check if period already exists
+        current_period = self.get_current_period(target_date)
+        if current_period:
+            return current_period['id']
+
+        # Create new monthly period for the date
+        year = target_date.year
+        month = target_date.month
+
+        try:
+            return self.create_monthly_period(year, month)
+        except ValueError as e:
+            # Period might already exist due to race condition, try to get it again
+            current_period = self.get_current_period(target_date)
+            if current_period:
+                return current_period['id']
+            raise e
\ No newline at end of file
diff --git a/tests/test_period_cli_commands.py b/tests/test_period_cli_commands.py
new file mode 100644
index 00000000..4761577c
--- /dev/null
+++ b/tests/test_period_cli_commands.py
@@ -0,0 +1,454 @@
+"""
+Tests for MarkiTect period management CLI commands.
+
+This module tests the command-line interface for period management including:
+- Period creation, listing, and status management
+- Period calculation and lifecycle operations
+- CLI error handling and validation
+"""
+
+import pytest
+import tempfile
+import os
+from datetime import date
+from decimal import Decimal
+from click.testing import CliRunner
+
+from markitect.finance.cli import cost_commands
+from markitect.finance.models import FinanceModels
+from markitect.finance.period_manager import PeriodManager
+from markitect.finance.cost_manager import CostItemManager, CostItem
+
+
+class TestPeriodCLICommands:
+    """Test suite for period management CLI commands."""
+
+    @pytest.fixture
+    def temp_db(self):
+        """Create temporary database for testing."""
+        fd, path = tempfile.mkstemp(suffix='.db')
+        os.close(fd)
+        yield path
+        os.unlink(path)
+
+    @pytest.fixture
+    def setup_test_data(self, temp_db):
+        """Setup test database with sample period data."""
+        finance_models = FinanceModels(temp_db)
+        finance_models.initialize_finance_schema()
+
+        period_manager = PeriodManager(temp_db)
+
+        # Create sample period
+        period_id = period_manager.create_period(
+            period_start=date(2025, 1, 1),
+            period_end=date(2025, 1, 31),
+            period_type='monthly'
+        )
+
+        return temp_db, period_id
+
+    @pytest.fixture
+    def runner(self):
+        """Create Click test runner."""
+        return CliRunner()
+
+    def test_period_create_success(self, runner, temp_db):
+        """Test period creation via CLI."""
+        # Initialize database first
+        finance_models = FinanceModels(temp_db)
+        finance_models.initialize_finance_schema()
+
+        result = runner.invoke(cost_commands, [
+            'period', 'create',
+            '--start-date', '2025-02-01',
+            '--end-date', '2025-02-28',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 0
+        assert "✅ Created period #" in result.output
+        assert "📅 Period: 2025-02-01 to 2025-02-28" in result.output
+        assert "📊 Type: monthly" in result.output
+
+    def test_period_create_with_loss_forward(self, runner, temp_db):
+        """Test period creation with loss carried forward."""
+        finance_models = FinanceModels(temp_db)
+        finance_models.initialize_finance_schema()
+
+        result = runner.invoke(cost_commands, [
+            'period', 'create',
+            '--start-date', '2025-03-01',
+            '--end-date', '2025-03-31',
+            '--loss-forward', '15.75',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 0
+        assert "✅ Created period #" in result.output
+        assert "💸 Loss carried forward: €15.7500" in result.output
+
+    def test_period_create_invalid_dates(self, runner, temp_db):
+        """Test period creation with invalid date format."""
+        finance_models = FinanceModels(temp_db)
+        finance_models.initialize_finance_schema()
+
+        result = runner.invoke(cost_commands, [
+            'period', 'create',
+            '--start-date', 'invalid-date',
+            '--end-date', '2025-02-28',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 1
+        assert "Error: Dates must be in YYYY-MM-DD format" in result.output
+
+    def test_period_create_overlapping_fails(self, runner, setup_test_data):
+        """Test that creating overlapping periods fails."""
+        temp_db, existing_period_id = setup_test_data
+
+        result = runner.invoke(cost_commands, [
+            'period', 'create',
+            '--start-date', '2025-01-15',  # Overlaps with existing period
+            '--end-date', '2025-02-15',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 1
+        assert "Error:" in result.output
+        assert "overlaps" in result.output.lower()
+
+    def test_period_list_all(self, runner, setup_test_data):
+        """Test listing all periods."""
+        temp_db, period_id = setup_test_data
+
+        result = runner.invoke(cost_commands, [
+            'period', 'list',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 0
+        assert "📅 Calculation Periods" in result.output
+        assert "2025-01-01" in result.output
+        assert "2025-01-31" in result.output
+        assert "Total: 1 periods" in result.output
+
+    def test_period_list_with_status_filter(self, runner, setup_test_data):
+        """Test listing periods with status filter."""
+        temp_db, period_id = setup_test_data
+
+        # Create second period and close it
+        period_manager = PeriodManager(temp_db)
+        period_id2 = period_manager.create_period(
+            period_start=date(2025, 2, 1),
+            period_end=date(2025, 2, 28)
+        )
+        period_manager.close_period(period_id2)
+
+        # Filter by open status
+        result = runner.invoke(cost_commands, [
+            'period', 'list',
+            '--status', 'open',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 0
+        assert "2025-01-01" in result.output  # First period should be shown
+        assert "2025-02-01" not in result.output  # Second period should be filtered out
+
+        # Filter by closed status
+        result = runner.invoke(cost_commands, [
+            'period', 'list',
+            '--status', 'closed',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 0
+        assert "2025-02-01" in result.output  # Second period should be shown
+        assert "2025-01-01" not in result.output  # First period should be filtered out
+
+    def test_period_list_with_date_filters(self, runner, temp_db):
+        """Test listing periods with date range filters."""
+        finance_models = FinanceModels(temp_db)
+        finance_models.initialize_finance_schema()
+
+        period_manager = PeriodManager(temp_db)
+
+        # Create periods in different months
+        jan_period = period_manager.create_period(date(2025, 1, 1), date(2025, 1, 31))
+        feb_period = period_manager.create_period(date(2025, 2, 1), date(2025, 2, 28))
+
+        # Filter by start date
+        result = runner.invoke(cost_commands, [
+            'period', 'list',
+            '--start-from', '2025-02-01',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 0
+        assert "2025-02-01" in result.output
+        assert "2025-01-01" not in result.output
+
+    def test_period_list_empty(self, runner, temp_db):
+        """Test listing periods when none exist."""
+        finance_models = FinanceModels(temp_db)
+        finance_models.initialize_finance_schema()
+
+        result = runner.invoke(cost_commands, [
+            'period', 'list',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 0
+        assert "No periods found matching criteria" in result.output
+
+    def test_period_show_details(self, runner, setup_test_data):
+        """Test showing period details."""
+        temp_db, period_id = setup_test_data
+
+        result = runner.invoke(cost_commands, [
+            'period', 'show', str(period_id),
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 0
+        assert f"📅 Period #{period_id} Details" in result.output
+        assert "Start Date: 2025-01-01" in result.output
+        assert "End Date: 2025-01-31" in result.output
+        assert "Type: monthly" in result.output
+        assert "Status: open" in result.output
+
+    def test_period_show_nonexistent(self, runner, temp_db):
+        """Test showing non-existent period."""
+        finance_models = FinanceModels(temp_db)
+        finance_models.initialize_finance_schema()
+
+        result = runner.invoke(cost_commands, [
+            'period', 'show', '999',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 1
+        assert "Period #999 not found" in result.output
+
+    def test_period_calculate(self, runner, setup_test_data):
+        """Test period cost calculation."""
+        temp_db, period_id = setup_test_data
+
+        # Add some cost items for calculation
+        cost_manager = CostItemManager(temp_db)
+        infra_cat = cost_manager.get_category_by_name('Infrastructure')
+
+        cost_item = CostItem(
+            category_id=infra_cat['id'],
+            name='Test Server',
+            cost_type='monthly',
+            amount_eur=Decimal('25.00'),
+            starting_from_date=date(2025, 1, 1)
+        )
+        cost_manager.create_cost_item(cost_item)
+
+        result = runner.invoke(cost_commands, [
+            'period', 'calculate', str(period_id),
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 0
+        assert f"📊 Period #{period_id} Cost Calculation" in result.output
+        assert "Period: 2025-01-01 to 2025-01-31" in result.output
+        assert "Monthly Recurring: €25.00" in result.output
+        assert "Total Period Cost: €25.00" in result.output
+
+    def test_period_calculate_nonexistent(self, runner, temp_db):
+        """Test calculating costs for non-existent period."""
+        finance_models = FinanceModels(temp_db)
+        finance_models.initialize_finance_schema()
+
+        result = runner.invoke(cost_commands, [
+            'period', 'calculate', '999',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 1
+        assert "Error calculating period:" in result.output
+
+    def test_period_status_update(self, runner, setup_test_data):
+        """Test period status update."""
+        temp_db, period_id = setup_test_data
+
+        result = runner.invoke(cost_commands, [
+            'period', 'status', str(period_id), 'calculating',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 0
+        assert f"✅ Period #{period_id} status updated to 'calculating'" in result.output
+
+        # Verify the status was actually updated
+        result = runner.invoke(cost_commands, [
+            'period', 'show', str(period_id),
+            '--database', temp_db
+        ])
+
+        assert "Status: calculating" in result.output
+
+    def test_period_status_update_invalid_status(self, runner, setup_test_data):
+        """Test period status update with invalid status."""
+        temp_db, period_id = setup_test_data
+
+        result = runner.invoke(cost_commands, [
+            'period', 'status', str(period_id), 'invalid',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 2  # Click validation error
+        assert "Invalid value" in result.output
+
+    def test_period_status_update_nonexistent(self, runner, temp_db):
+        """Test status update for non-existent period."""
+        finance_models = FinanceModels(temp_db)
+        finance_models.initialize_finance_schema()
+
+        result = runner.invoke(cost_commands, [
+            'period', 'status', '999', 'calculating',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 1
+        assert "Error:" in result.output
+
+    def test_period_close(self, runner, setup_test_data):
+        """Test period closure."""
+        temp_db, period_id = setup_test_data
+
+        result = runner.invoke(cost_commands, [
+            'period', 'close', str(period_id),
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 0
+        assert f"✅ Period #{period_id} has been closed" in result.output
+        assert "💰 Final total cost:" in result.output
+
+        # Verify the period is actually closed
+        result = runner.invoke(cost_commands, [
+            'period', 'show', str(period_id),
+            '--database', temp_db
+        ])
+
+        assert "Status: closed" in result.output
+
+    def test_period_close_nonexistent(self, runner, temp_db):
+        """Test closing non-existent period."""
+        finance_models = FinanceModels(temp_db)
+        finance_models.initialize_finance_schema()
+
+        result = runner.invoke(cost_commands, [
+            'period', 'close', '999',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 1
+        assert "Error:" in result.output
+
+    def test_period_current_exists(self, runner, setup_test_data):
+        """Test finding current period when it exists."""
+        temp_db, period_id = setup_test_data
+
+        result = runner.invoke(cost_commands, [
+            'period', 'current',
+            '--date', '2025-01-15',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 0
+        assert "📅 Current Active Period" in result.output
+        assert f"Period #{period_id}" in result.output
+        assert "Dates: 2025-01-01 to 2025-01-31" in result.output
+
+    def test_period_current_not_found(self, runner, temp_db):
+        """Test finding current period when none exists."""
+        finance_models = FinanceModels(temp_db)
+        finance_models.initialize_finance_schema()
+
+        result = runner.invoke(cost_commands, [
+            'period', 'current',
+            '--date', '2025-03-15',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 0
+        assert "No active period found for 2025-03-15" in result.output
+
+    def test_period_current_default_to_today(self, runner, temp_db):
+        """Test current period defaults to today."""
+        finance_models = FinanceModels(temp_db)
+        finance_models.initialize_finance_schema()
+
+        result = runner.invoke(cost_commands, [
+            'period', 'current',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 0
+        assert "No active period found for today" in result.output
+        assert "💡 Create one with:" in result.output
+        assert "markitect cost period create" in result.output
+
+    def test_period_current_invalid_date(self, runner, temp_db):
+        """Test current period with invalid date format."""
+        finance_models = FinanceModels(temp_db)
+        finance_models.initialize_finance_schema()
+
+        result = runner.invoke(cost_commands, [
+            'period', 'current',
+            '--date', 'invalid-date',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 1
+        assert "Error: Date must be in YYYY-MM-DD format" in result.output
+
+    def test_period_help_commands(self, runner):
+        """Test help output for period commands."""
+        # Test main period help
+        result = runner.invoke(cost_commands, ['period', '--help'])
+        assert result.exit_code == 0
+        assert "Manage calculation periods and lifecycle" in result.output
+
+        # Test create help
+        result = runner.invoke(cost_commands, ['period', 'create', '--help'])
+        assert result.exit_code == 0
+        assert "Create a new calculation period" in result.output
+
+        # Test list help
+        result = runner.invoke(cost_commands, ['period', 'list', '--help'])
+        assert result.exit_code == 0
+        assert "List calculation periods with optional filtering" in result.output
+
+    def test_period_commands_missing_database(self, runner):
+        """Test period commands without database specification."""
+        # These should use default config path and still work or show appropriate error
+        result = runner.invoke(cost_commands, [
+            'period', 'list'
+        ])
+
+        # Should succeed with default database configuration
+        assert result.exit_code == 0
+
+    def test_period_create_quarterly_type(self, runner, temp_db):
+        """Test creating quarterly period type."""
+        finance_models = FinanceModels(temp_db)
+        finance_models.initialize_finance_schema()
+
+        result = runner.invoke(cost_commands, [
+            'period', 'create',
+            '--start-date', '2025-04-01',
+            '--end-date', '2025-06-30',
+            '--type', 'quarterly',
+            '--database', temp_db
+        ])
+
+        assert result.exit_code == 0
+        assert "✅ Created period #" in result.output
+        assert "📊 Type: quarterly" in result.output
\ No newline at end of file
diff --git a/tests/test_period_manager.py b/tests/test_period_manager.py
new file mode 100644
index 00000000..d7c8ebec
--- /dev/null
+++ b/tests/test_period_manager.py
@@ -0,0 +1,489 @@
+"""
+Tests for MarkiTect Period Management Framework.
+
+This module tests the complete period lifecycle management system including:
+- Period creation, status management, and lifecycle transitions
+- Period overlap validation and conflict resolution
+- Period calculations and cost aggregation
+- Period closure validation and audit trails
+- Current period detection and auto-creation
+"""
+
+import pytest
+import tempfile
+import os
+from datetime import date, datetime, timedelta
+from decimal import Decimal
+
+from markitect.finance.period_manager import PeriodManager, PeriodStatus, Period
+from markitect.finance.models import FinanceModels
+from markitect.finance.cost_manager import CostItemManager, CostItem
+
+
+class TestPeriodManager:
+    """Test suite for period management system."""
+
+    @pytest.fixture
+    def temp_db(self):
+        """Create temporary database for testing."""
+        fd, path = tempfile.mkstemp(suffix='.db')
+        os.close(fd)
+        yield path
+        os.unlink(path)
+
+    @pytest.fixture
+    def period_manager(self, temp_db):
+        """Create period manager with initialized database."""
+        finance_models = FinanceModels(temp_db)
+        finance_models.initialize_finance_schema()
+        return PeriodManager(temp_db)
+
+    @pytest.fixture
+    def sample_period_data(self):
+        """Sample period data for testing."""
+        return {
+            'period_start': date(2025, 1, 1),
+            'period_end': date(2025, 1, 31),
+            'period_type': 'monthly'
+        }
+
+    def test_period_status_enum(self):
+        """Test period status enumeration."""
+        assert PeriodStatus.OPEN.value == 'open'
+        assert PeriodStatus.CALCULATING.value == 'calculating'
+        assert PeriodStatus.CLOSED.value == 'closed'
+
+    def test_period_dataclass(self):
+        """Test Period dataclass creation."""
+        period = Period(
+            id=1,
+            period_start=date(2025, 1, 1),
+            period_end=date(2025, 1, 31),
+            period_type='monthly',
+            status='open',
+            total_costs=Decimal('100.50')
+        )
+
+        assert period.id == 1
+        assert period.period_start == date(2025, 1, 1)
+        assert period.period_end == date(2025, 1, 31)
+        assert period.total_costs == Decimal('100.50')
+
+    def test_create_period_success(self, period_manager, sample_period_data):
+        """Test successful period creation."""
+        period_id = period_manager.create_period(
+            period_start=sample_period_data['period_start'],
+            period_end=sample_period_data['period_end'],
+            period_type=sample_period_data['period_type']
+        )
+
+        assert period_id is not None
+        assert isinstance(period_id, int)
+
+        # Verify period was created
+        created_period = period_manager.get_period_by_id(period_id)
+        assert created_period is not None
+        assert created_period['period_start'] == sample_period_data['period_start'].isoformat()
+        assert created_period['period_end'] == sample_period_data['period_end'].isoformat()
+        assert created_period['status'] == PeriodStatus.OPEN.value
+
+    def test_create_period_invalid_dates(self, period_manager):
+        """Test period creation with invalid date range."""
+        with pytest.raises(ValueError, match="Period end date must be after start date"):
+            period_manager.create_period(
+                period_start=date(2025, 1, 31),
+                period_end=date(2025, 1, 1)  # End before start
+            )
+
+    def test_create_period_with_loss_carried_forward(self, period_manager, sample_period_data):
+        """Test period creation with loss carried forward."""
+        loss_amount = Decimal('25.50')
+        period_id = period_manager.create_period(
+            period_start=sample_period_data['period_start'],
+            period_end=sample_period_data['period_end'],
+            loss_carried_forward=loss_amount
+        )
+
+        created_period = period_manager.get_period_by_id(period_id)
+        assert created_period['loss_carried_forward'] == loss_amount
+
+    def test_find_overlapping_periods(self, period_manager, sample_period_data):
+        """Test overlap detection functionality."""
+        # Create first period
+        period_id1 = period_manager.create_period(
+            period_start=sample_period_data['period_start'],
+            period_end=sample_period_data['period_end']
+        )
+
+        # Test overlapping period detection
+        overlapping = period_manager.find_overlapping_periods(
+            period_start=date(2025, 1, 15),  # Overlaps with existing
+            period_end=date(2025, 2, 15)
+        )
+
+        assert len(overlapping) == 1
+        assert overlapping[0]['id'] == period_id1
+
+    def test_create_overlapping_period_fails(self, period_manager, sample_period_data):
+        """Test that creating overlapping periods fails."""
+        # Create first period
+        period_manager.create_period(
+            period_start=sample_period_data['period_start'],
+            period_end=sample_period_data['period_end']
+        )
+
+        # Try to create overlapping period
+        with pytest.raises(ValueError, match="Period overlaps with existing periods"):
+            period_manager.create_period(
+                period_start=date(2025, 1, 15),  # Overlaps
+                period_end=date(2025, 2, 15)
+            )
+
+    def test_update_period_status_valid_transition(self, period_manager, sample_period_data):
+        """Test valid period status transitions."""
+        period_id = period_manager.create_period(
+            period_start=sample_period_data['period_start'],
+            period_end=sample_period_data['period_end']
+        )
+
+        # Transition from OPEN to CALCULATING
+        success = period_manager.update_period_status(period_id, PeriodStatus.CALCULATING.value)
+        assert success is True
+
+        updated_period = period_manager.get_period_by_id(period_id)
+        assert updated_period['status'] == PeriodStatus.CALCULATING.value
+
+        # Transition from CALCULATING to CLOSED
+        success = period_manager.update_period_status(period_id, PeriodStatus.CLOSED.value)
+        assert success is True
+
+        updated_period = period_manager.get_period_by_id(period_id)
+        assert updated_period['status'] == PeriodStatus.CLOSED.value
+
+    def test_update_period_status_invalid_status(self, period_manager, sample_period_data):
+        """Test update with invalid status."""
+        period_id = period_manager.create_period(
+            period_start=sample_period_data['period_start'],
+            period_end=sample_period_data['period_end']
+        )
+
+        with pytest.raises(ValueError, match="Invalid status 'invalid'"):
+            period_manager.update_period_status(period_id, 'invalid')
+
+    def test_update_period_status_nonexistent_period(self, period_manager):
+        """Test update status for non-existent period."""
+        with pytest.raises(ValueError, match="Period #999 not found"):
+            period_manager.update_period_status(999, PeriodStatus.CALCULATING.value)
+
+    def test_calculate_period_costs(self, period_manager, sample_period_data, temp_db):
+        """Test period cost calculation functionality."""
+        # Create period
+        period_id = period_manager.create_period(
+            period_start=sample_period_data['period_start'],
+            period_end=sample_period_data['period_end']
+        )
+
+        # Set up cost manager and add test data
+        finance_models = FinanceModels(temp_db)
+        cost_manager = CostItemManager(temp_db)
+
+        # Get categories
+        infra_cat = cost_manager.get_category_by_name('Infrastructure')
+        software_cat = cost_manager.get_category_by_name('Software')
+
+        # Create test cost items
+        monthly_item = CostItem(
+            category_id=infra_cat['id'],
+            name='Monthly Server',
+            cost_type='monthly',
+            amount_eur=Decimal('25.00'),
+            starting_from_date=date(2024, 12, 1)  # Started before period
+        )
+
+        one_time_item = CostItem(
+            category_id=software_cat['id'],
+            name='One-time License',
+            cost_type='one_time',
+            amount_eur=Decimal('50.00'),
+            starting_from_date=date(2025, 1, 15)  # Within period
+        )
+
+        cost_manager.create_cost_item(monthly_item)
+        cost_manager.create_cost_item(one_time_item)
+
+        # Calculate period costs
+        calculation_result = period_manager.calculate_period_costs(period_id)
+
+        # Verify calculation results
+        assert calculation_result['period_id'] == period_id
+        assert calculation_result['monthly_costs'] == 25.0
+        assert calculation_result['one_time_costs'] == 50.0
+        assert calculation_result['total_costs'] == 75.0
+
+        # Verify period was updated
+        updated_period = period_manager.get_period_by_id(period_id)
+        assert updated_period['total_costs'] == Decimal('75.00')
+
+    def test_close_period(self, period_manager, sample_period_data):
+        """Test period closure functionality."""
+        period_id = period_manager.create_period(
+            period_start=sample_period_data['period_start'],
+            period_end=sample_period_data['period_end']
+        )
+
+        # Close the period
+        success = period_manager.close_period(period_id)
+        assert success is True
+
+        # Verify period is closed
+        closed_period = period_manager.get_period_by_id(period_id)
+        assert closed_period['status'] == PeriodStatus.CLOSED.value
+
+    def test_close_period_already_closed(self, period_manager, sample_period_data):
+        """Test closing an already closed period."""
+        period_id = period_manager.create_period(
+            period_start=sample_period_data['period_start'],
+            period_end=sample_period_data['period_end']
+        )
+
+        # Close period first time
+        period_manager.close_period(period_id)
+
+        # Close again (should succeed without error)
+        success = period_manager.close_period(period_id)
+        assert success is True
+
+    def test_close_nonexistent_period(self, period_manager):
+        """Test closing non-existent period."""
+        with pytest.raises(ValueError, match="Period #999 not found"):
+            period_manager.close_period(999)
+
+    def test_list_periods_no_filter(self, period_manager, sample_period_data):
+        """Test listing all periods without filters."""
+        # Create multiple periods
+        period_id1 = period_manager.create_period(
+            period_start=date(2025, 1, 1),
+            period_end=date(2025, 1, 31)
+        )
+
+        period_id2 = period_manager.create_period(
+            period_start=date(2025, 2, 1),
+            period_end=date(2025, 2, 28)
+        )
+
+        # List all periods
+        periods = period_manager.list_periods()
+
+        assert len(periods) == 2
+        period_ids = [p['id'] for p in periods]
+        assert period_id1 in period_ids
+        assert period_id2 in period_ids
+
+    def test_list_periods_with_status_filter(self, period_manager):
+        """Test listing periods with status filter."""
+        # Create periods with different statuses
+        period_id1 = period_manager.create_period(
+            period_start=date(2025, 1, 1),
+            period_end=date(2025, 1, 31)
+        )
+
+        period_id2 = period_manager.create_period(
+            period_start=date(2025, 2, 1),
+            period_end=date(2025, 2, 28)
+        )
+
+        # Close one period
+        period_manager.close_period(period_id2)
+
+        # Filter by open status
+        open_periods = period_manager.list_periods(status_filter=PeriodStatus.OPEN.value)
+        assert len(open_periods) == 1
+        assert open_periods[0]['id'] == period_id1
+
+        # Filter by closed status
+        closed_periods = period_manager.list_periods(status_filter=PeriodStatus.CLOSED.value)
+        assert len(closed_periods) == 1
+        assert closed_periods[0]['id'] == period_id2
+
+    def test_list_periods_with_date_filters(self, period_manager):
+        """Test listing periods with date range filters."""
+        # Create periods in different months
+        jan_period = period_manager.create_period(
+            period_start=date(2025, 1, 1),
+            period_end=date(2025, 1, 31)
+        )
+
+        feb_period = period_manager.create_period(
+            period_start=date(2025, 2, 1),
+            period_end=date(2025, 2, 28)
+        )
+
+        # Filter by start date
+        periods_from_feb = period_manager.list_periods(start_date=date(2025, 2, 1))
+        assert len(periods_from_feb) == 1
+        assert periods_from_feb[0]['id'] == feb_period
+
+        # Filter by end date
+        periods_until_jan = period_manager.list_periods(end_date=date(2025, 1, 31))
+        assert len(periods_until_jan) == 1
+        assert periods_until_jan[0]['id'] == jan_period
+
+    def test_get_current_period(self, period_manager):
+        """Test getting current period for a specific date."""
+        # Create period covering January 2025
+        period_id = period_manager.create_period(
+            period_start=date(2025, 1, 1),
+            period_end=date(2025, 1, 31)
+        )
+
+        # Test date within period
+        current = period_manager.get_current_period(date(2025, 1, 15))
+        assert current is not None
+        assert current['id'] == period_id
+
+        # Test date outside period
+        current = period_manager.get_current_period(date(2025, 2, 15))
+        assert current is None
+
+    def test_get_current_period_defaults_to_today(self, period_manager):
+        """Test that get_current_period defaults to today's date."""
+        today = date.today()
+
+        # Create period covering today
+        period_id = period_manager.create_period(
+            period_start=date(today.year, today.month, 1),
+            period_end=date(today.year, today.month, 31) if today.month != 12
+                   else date(today.year, 12, 31)
+        )
+
+        # Get current period without specifying date
+        current = period_manager.get_current_period()
+        assert current is not None
+        assert current['id'] == period_id
+
+    def test_create_monthly_period(self, period_manager):
+        """Test convenience method for creating monthly periods."""
+        period_id = period_manager.create_monthly_period(2025, 3)
+        assert period_id is not None
+
+        # Verify correct dates were set
+        period = period_manager.get_period_by_id(period_id)
+        assert period['period_start'] == '2025-03-01'
+        assert period['period_end'] == '2025-03-31'
+        assert period['period_type'] == 'monthly'
+
+    def test_create_monthly_period_december(self, period_manager):
+        """Test creating monthly period for December (year boundary)."""
+        period_id = period_manager.create_monthly_period(2025, 12)
+
+        period = period_manager.get_period_by_id(period_id)
+        assert period['period_start'] == '2025-12-01'
+        assert period['period_end'] == '2025-12-31'
+
+    def test_auto_create_period_for_date(self, period_manager):
+        """Test automatic period creation for a given date."""
+        test_date = date(2025, 5, 15)
+
+        # First call should create new period
+        period_id = period_manager.auto_create_period_for_date(test_date)
+        assert period_id is not None
+
+        # Second call should return existing period
+        period_id2 = period_manager.auto_create_period_for_date(test_date)
+        assert period_id2 == period_id
+
+        # Verify period covers the test date
+        period = period_manager.get_period_by_id(period_id)
+        assert period['period_start'] == '2025-05-01'
+        assert period['period_end'] == '2025-05-31'
+
+    def test_period_calculation_with_loss_carried_forward(self, period_manager, temp_db):
+        """Test period calculation including loss carried forward."""
+        # Create period with loss carried forward
+        period_id = period_manager.create_period(
+            period_start=date(2025, 1, 1),
+            period_end=date(2025, 1, 31),
+            loss_carried_forward=Decimal('15.75')
+        )
+
+        # Add a cost item
+        cost_manager = CostItemManager(temp_db)
+        infra_cat = cost_manager.get_category_by_name('Infrastructure')
+
+        cost_item = CostItem(
+            category_id=infra_cat['id'],
+            name='Test Server',
+            cost_type='monthly',
+            amount_eur=Decimal('10.00'),
+            starting_from_date=date(2025, 1, 1)
+        )
+        cost_manager.create_cost_item(cost_item)
+
+        # Calculate costs
+        calculation = period_manager.calculate_period_costs(period_id)
+
+        # Should include loss carried forward
+        assert calculation['loss_carried_forward'] == 15.75
+        assert calculation['monthly_costs'] == 10.0
+        assert calculation['total_costs'] == 25.75  # 10.0 + 15.75
+
+    def test_period_cost_calculation_edge_cases(self, period_manager, temp_db):
+        """Test period cost calculation with various edge cases."""
+        # Create period
+        period_id = period_manager.create_period(
+            period_start=date(2025, 3, 1),
+            period_end=date(2025, 3, 31)
+        )
+
+        cost_manager = CostItemManager(temp_db)
+        infra_cat = cost_manager.get_category_by_name('Infrastructure')
+
+        # Item that starts before period and ends during period
+        item1 = CostItem(
+            category_id=infra_cat['id'],
+            name='Ending Item',
+            cost_type='monthly',
+            amount_eur=Decimal('20.00'),
+            starting_from_date=date(2025, 1, 1),
+            ending_date=date(2025, 3, 15)
+        )
+
+        # Item that starts after period
+        item2 = CostItem(
+            category_id=infra_cat['id'],
+            name='Future Item',
+            cost_type='monthly',
+            amount_eur=Decimal('30.00'),
+            starting_from_date=date(2025, 4, 1)
+        )
+
+        # One-time item outside period
+        item3 = CostItem(
+            category_id=infra_cat['id'],
+            name='Past One-time',
+            cost_type='one_time',
+            amount_eur=Decimal('100.00'),
+            starting_from_date=date(2025, 2, 15)
+        )
+
+        cost_manager.create_cost_item(item1)
+        cost_manager.create_cost_item(item2)
+        cost_manager.create_cost_item(item3)
+
+        # Calculate costs
+        calculation = period_manager.calculate_period_costs(period_id)
+
+        # Only item1 should be included (ends during period)
+        assert calculation['monthly_costs'] == 20.0
+        assert calculation['one_time_costs'] == 0.0
+        assert calculation['total_costs'] == 20.0
+
+    def test_error_handling_database_errors(self, period_manager):
+        """Test error handling for database-related issues."""
+        # Test with invalid period ID
+        with pytest.raises(ValueError, match="Period #-1 not found"):
+            period_manager.calculate_period_costs(-1)
+
+        # Test getting non-existent period
+        result = period_manager.get_period_by_id(99999)
+        assert result is None
\ No newline at end of file