diff --git a/tests/test_config.py b/tests/test_config.py
new file mode 100644
index 0000000..542070b
--- /dev/null
+++ b/tests/test_config.py
@@ -0,0 +1,111 @@
+"""Tests for configuration system."""
+
+import pytest
+import json
+import tempfile
+import os
+
+from config.simulation_config import SimulationConfig, HeadlessConfig, InteractiveConfig, ExperimentConfig
+from config.config_loader import ConfigLoader
+
+
+class TestSimulationConfig:
+    """Test simulation configuration."""
+
+    def test_custom_values(self):
+        """Test custom configuration values."""
+        config = SimulationConfig(
+            grid_width=50,
+            grid_height=40,
+            cell_size=15,
+            initial_cells=100,
+            initial_food=200,
+            food_spawning=False,
+            random_seed=999,
+            default_tps=120.0
+        )
+
+        assert config.grid_width == 50
+        assert config.grid_height == 40
+        assert config.cell_size == 15
+        assert config.initial_cells == 100
+        assert config.initial_food == 200
+        assert config.food_spawning == False
+        assert config.random_seed == 999
+        assert config.default_tps == 120.0
+
+
+class TestConfigLoader:
+    """Test configuration loader."""
+
+    def test_load_headless_config(self):
+        """Test loading headless configuration from file."""
+        # Create a temporary file
+        with tempfile.NamedTemporaryFile(mode='w', suffix='.json', delete=False) as f:
+            temp_file = f.name
+            json.dump({
+                "max_ticks": 10000,
+                "max_duration": 300.0,
+                "output": {
+                    "enabled": True,
+                    "directory": "test_output",
+                    "formats": ["json", "csv"]
+                },
+                "simulation": {
+                    "grid_width": 50,
+                    "grid_height": 40,
+                    "initial_cells": 100,
+                    "default_tps": 60.0
+                }
+            }, f)
+
+        try:
+            config = ConfigLoader.load_headless_config(temp_file)
+
+            assert config.max_ticks == 10000
+            assert config.max_duration == 300.0
+            assert config.output.enabled == True
+            assert config.output.directory == "test_output"
+            assert config.output.formats == ["json", "csv"]
+            assert config.simulation.grid_width == 50
+            assert config.simulation.grid_height == 40
+            assert config.simulation.initial_cells == 100
+            assert config.simulation.default_tps == 60.0
+        finally:
+            os.unlink(temp_file)
+
+    def test_save_load_json(self):
+        """Test saving and loading configuration from JSON file."""
+        # Create a temporary file
+        with tempfile.NamedTemporaryFile(mode='w', suffix='.json', delete=False) as f:
+            temp_file = f.name
+
+        try:
+            # Create test configuration
+            original_config = HeadlessConfig(
+                max_ticks=5000,
+                simulation=SimulationConfig(
+                    grid_width=25,
+                    grid_height=20,
+                    initial_cells=75,
+                    default_tps=90.0
+                )
+            )
+
+            # Save to JSON
+            ConfigLoader.save_config(original_config, temp_file)
+
+            # Load from JSON
+            loaded_config = ConfigLoader.load_headless_config(temp_file)
+
+            # Verify loaded config matches original
+            assert loaded_config.max_ticks == original_config.max_ticks
+            assert loaded_config.simulation.grid_width == original_config.simulation.grid_width
+            assert loaded_config.simulation.grid_height == original_config.simulation.grid_height
+            assert loaded_config.simulation.initial_cells == original_config.simulation.initial_cells
+            assert loaded_config.simulation.default_tps == original_config.simulation.default_tps
+
+        finally:
+            # Clean up temporary file
+            if os.path.exists(temp_file):
+                os.unlink(temp_file)
\ No newline at end of file
diff --git a/tests/test_core_components.py b/tests/test_core_components.py
new file mode 100644
index 0000000..25febc4
--- /dev/null
+++ b/tests/test_core_components.py
@@ -0,0 +1,390 @@
+"""Tests for core simulation components."""
+
+import pytest
+import time
+from unittest.mock import Mock
+
+from core.simulation_core import SimulationCore, SimulationState
+from core.timing import TimingController, TimingState
+from core.event_bus import EventBus, EventType, Event
+from config.simulation_config import SimulationConfig
+
+
+class TestTimingController:
+    """Test timing controller functionality."""
+
+    def test_initialization(self):
+        """Test timing controller initialization."""
+        event_bus = Mock()
+        timing = TimingController(default_tps=60.0, event_bus=event_bus)
+
+        assert timing.state.tps == 60.0
+        assert timing.state.is_paused == False
+        assert timing.state.sprint_mode == False
+        assert timing.state.speed_multiplier == 1.0
+        assert timing.default_tps == 60.0
+        assert timing.total_ticks == 0
+
+    def test_set_tps(self):
+        """Test TPS setting."""
+        timing = TimingController(default_tps=60.0)
+
+        timing.set_tps(120.0)
+        assert timing.state.tps == 120.0
+        assert timing.state.speed_multiplier == 2.0
+
+        # Test bounds
+        timing.set_tps(2000.0)  # Should be capped at 1000
+        assert timing.state.tps == 1000.0
+
+        timing.set_tps(0.5)  # Should be capped at 1.0
+        assert timing.state.tps == 1.0
+
+    def test_set_speed_multiplier(self):
+        """Test speed multiplier setting."""
+        timing = TimingController(default_tps=60.0)
+
+        timing.set_speed_multiplier(2.0)
+        assert timing.state.speed_multiplier == 2.0
+        assert timing.state.tps == 120.0
+
+        # Test bounds
+        timing.set_speed_multiplier(20.0)  # Should be capped at 10.0
+        assert timing.state.speed_multiplier == 10.0
+
+        timing.set_speed_multiplier(0.05)  # Should be capped at 0.1
+        assert timing.state.speed_multiplier == 0.1
+
+    def test_pause_toggle(self):
+        """Test pause functionality."""
+        timing = TimingController()
+
+        assert timing.state.is_paused == False
+
+        timing.toggle_pause()
+        assert timing.state.is_paused == True
+
+        timing.set_pause(False)  # This will unpause since it's different
+        assert timing.state.is_paused == False
+
+        timing.set_pause(True)  # This will pause since it's different
+        assert timing.state.is_paused == True
+
+        timing.toggle_pause()
+        assert timing.state.is_paused == False
+
+    def test_sprint_mode(self):
+        """Test sprint mode functionality."""
+        timing = TimingController()
+
+        assert timing.state.sprint_mode == False
+
+        timing.toggle_sprint_mode()
+        assert timing.state.sprint_mode == True
+        assert timing.sprint_start_time is not None
+
+        timing.toggle_sprint_mode()
+        assert timing.state.sprint_mode == False
+        assert timing.sprint_start_time is None
+
+    def test_should_tick(self):
+        """Test tick timing logic."""
+        timing = TimingController(default_tps=60.0)
+
+        # When paused, should not tick
+        timing.state.is_paused = True
+        assert timing.should_tick() == False
+
+        # When in sprint mode, should always tick
+        timing.state.is_paused = False
+        timing.state.sprint_mode = True
+        assert timing.should_tick() == True
+
+        # Normal mode - should tick based on time
+        timing.state.sprint_mode = False
+        timing.last_tick_time = time.perf_counter()
+        time.sleep(0.02)  # 20ms, should be enough for 60 TPS (16.67ms interval)
+        assert timing.should_tick() == True
+
+    def test_update_timing(self):
+        """Test timing update calculations."""
+        timing = TimingController(default_tps=60.0)
+
+        initial_ticks = timing.total_ticks
+        timing.update_timing()
+
+        assert timing.total_ticks == initial_ticks + 1
+        assert timing.tick_counter == 1
+
+    def test_display_functions(self):
+        """Test display utility functions."""
+        timing = TimingController(default_tps=60.0)
+
+        # Test TPS display
+        timing.actual_tps = 59.7
+        assert timing.get_display_tps() == 60
+
+        # Test speed display
+        assert timing.get_current_speed_display() == "1x"
+
+        timing.state.is_paused = True
+        assert timing.get_current_speed_display() == "Paused"
+
+        timing.state.is_paused = False
+        timing.state.sprint_mode = True
+        assert timing.get_current_speed_display() == "Sprint"
+
+        timing.state.sprint_mode = False
+        timing.state.speed_multiplier = 2.0
+        assert timing.get_current_speed_display() == "2.0x"
+
+
+class TestEventBus:
+    """Test event bus functionality."""
+
+    def test_initialization(self):
+        """Test event bus initialization."""
+        bus = EventBus()
+        assert len(bus._subscribers) == 0
+        assert len(bus._event_history) == 0
+
+    def test_subscribe_unsubscribe(self):
+        """Test subscribing and unsubscribing to events."""
+        bus = EventBus()
+        callback = Mock()
+
+        bus.subscribe(EventType.SIMULATION_STATE_CHANGED, callback)
+        assert EventType.SIMULATION_STATE_CHANGED in bus._subscribers
+        assert callback in bus._subscribers[EventType.SIMULATION_STATE_CHANGED]
+
+        bus.unsubscribe(EventType.SIMULATION_STATE_CHANGED, callback)
+        assert callback not in bus._subscribers[EventType.SIMULATION_STATE_CHANGED]
+
+    def test_publish_and_receive(self):
+        """Test publishing and receiving events."""
+        bus = EventBus()
+        callback = Mock()
+
+        bus.subscribe(EventType.WORLD_TICK_COMPLETED, callback)
+
+        event = Event(type=EventType.WORLD_TICK_COMPLETED, data={'tick_count': 100})
+        bus.publish(event)
+
+        callback.assert_called_once_with(event)
+        assert len(bus._event_history) == 1
+        assert bus._event_history[0] == event
+
+    def test_multiple_subscribers(self):
+        """Test multiple subscribers to same event."""
+        bus = EventBus()
+        callback1 = Mock()
+        callback2 = Mock()
+
+        bus.subscribe(EventType.ENTITY_ADDED, callback1)
+        bus.subscribe(EventType.ENTITY_ADDED, callback2)
+
+        event = Event(type=EventType.ENTITY_ADDED, data={'entity_id': 123})
+        bus.publish(event)
+
+        callback1.assert_called_once_with(event)
+        callback2.assert_called_once_with(event)
+
+    def test_event_history(self):
+        """Test event history management."""
+        bus = EventBus()
+
+        # Add some events
+        for i in range(5):
+            event = Event(type=EventType.TIMING_UPDATE, data={'tick': i})
+            bus.publish(event)
+
+        assert len(bus._event_history) == 5
+
+        # Test getting recent events
+        recent = bus.get_recent_events(count=3)
+        assert len(recent) == 3
+        assert recent[0].data['tick'] == 2  # Third from last
+        assert recent[2].data['tick'] == 4  # Most recent
+
+        # Test filtered events
+        timing_events = bus.get_recent_events(event_type=EventType.TIMING_UPDATE)
+        assert len(timing_events) == 5
+
+        # Clear history
+        bus.clear_history()
+        assert len(bus._event_history) == 0
+
+    def test_callback_error_handling(self):
+        """Test that callback errors don't crash the event bus."""
+        bus = EventBus()
+
+        good_callback = Mock()
+
+        def bad_callback(event):
+            raise ValueError("Test error")
+
+        bus.subscribe(EventType.SIMULATION_STATE_CHANGED, good_callback)
+        bus.subscribe(EventType.SIMULATION_STATE_CHANGED, bad_callback)
+
+        # This should not raise an exception despite the bad callback
+        event = Event(type=EventType.SIMULATION_STATE_CHANGED, data={})
+        bus.publish(event)  # Should not crash
+
+        # Good callback should still be called
+        good_callback.assert_called_once()
+
+
+class TestSimulationCore:
+    """Test simulation core functionality."""
+
+    def test_initialization(self):
+        """Test simulation core initialization."""
+        config = SimulationConfig(
+            grid_width=20,
+            grid_height=15,
+            initial_cells=5,
+            initial_food=10
+        )
+        core = SimulationCore(config)
+
+        assert core.config == config
+        assert core.world is not None
+        assert core.timing is not None
+        assert core.event_bus is not None
+        assert core.state.total_ticks == 0
+        assert core.is_running == False
+
+    def test_start_stop(self):
+        """Test starting and stopping simulation."""
+        config = SimulationConfig(initial_cells=0, initial_food=0)
+        core = SimulationCore(config)
+
+        assert core.is_running == False
+        assert core.state.is_running == False
+
+        core.start()
+        assert core.is_running == True
+        assert core.state.is_running == True
+
+        core.stop()
+        assert core.is_running == False
+        assert core.state.is_running == False
+
+    def test_pause_resume(self):
+        """Test pause and resume functionality."""
+        config = SimulationConfig(initial_cells=0, initial_food=0)
+        core = SimulationCore(config)
+
+        core.start()
+
+        # Pause
+        core.pause()
+        assert core.timing.state.is_paused == True
+
+        # Resume
+        core.resume()
+        assert core.timing.state.is_paused == False
+
+        core.stop()
+
+    def test_step_execution(self):
+        """Test single step execution."""
+        config = SimulationConfig(initial_cells=0, initial_food=0)
+        core = SimulationCore(config)
+
+        initial_ticks = core.state.total_ticks
+        core.step()
+
+        # Should have advanced by exactly one tick
+        assert core.state.total_ticks == initial_ticks + 1
+
+    def test_tps_control(self):
+        """Test TPS control functionality."""
+        config = SimulationConfig(initial_cells=0, initial_food=0)
+        core = SimulationCore(config)
+
+        core.set_tps(120.0)
+        assert core.timing.state.tps == 120.0
+
+        core.set_speed_multiplier(2.0)
+        assert core.timing.state.speed_multiplier == 2.0
+
+    def test_entity_queries(self):
+        """Test entity query methods."""
+        config = SimulationConfig(
+            grid_width=10,
+            grid_height=10,
+            initial_cells=5,
+            initial_food=3
+        )
+        core = SimulationCore(config)
+
+        # Test entity counting
+        from world.objects import DefaultCell, FoodObject
+        cell_count = core.count_entities_by_type(DefaultCell)
+        food_count = core.count_entities_by_type(FoodObject)
+
+        assert cell_count == 5
+        assert food_count == 3
+
+        # Test position queries
+        from world.world import Position
+        position = Position(x=0, y=0)
+        entities_in_radius = core.get_entities_in_radius(position, radius=50)
+        assert len(entities_in_radius) >= 0  # Should find some entities
+
+    def test_world_state(self):
+        """Test world state collection."""
+        config = SimulationConfig(initial_cells=2, initial_food=3)
+        core = SimulationCore(config)
+
+        state = core.get_world_state()
+
+        assert 'tick_count' in state
+        assert 'actual_tps' in state
+        assert 'entity_counts' in state
+        assert state['entity_counts']['total'] == 5  # 2 cells + 3 food
+        assert state['entity_counts']['cells'] == 2
+        assert state['entity_counts']['food'] == 3
+
+    def test_entity_states(self):
+        """Test entity state collection."""
+        config = SimulationConfig(initial_cells=2, initial_food=1)
+        core = SimulationCore(config)
+
+        entities = core.get_entity_states()
+
+        assert len(entities) == 3  # 2 cells + 1 food
+
+        # Check cell entities
+        cell_entities = [e for e in entities if e['type'] == 'cell']
+        assert len(cell_entities) == 2
+
+        for cell in cell_entities:
+            assert 'id' in cell
+            assert 'position' in cell
+            assert 'energy' in cell
+            assert 'age' in cell
+            assert 'neural_network' in cell
+
+        # Check food entities
+        food_entities = [e for e in entities if e['type'] == 'food']
+        assert len(food_entities) == 1
+
+        for food in food_entities:
+            assert 'id' in food
+            assert 'position' in food
+            assert 'decay' in food
+
+    def test_sprint_mode(self):
+        """Test sprint mode functionality."""
+        config = SimulationConfig(initial_cells=0, initial_food=0)
+        core = SimulationCore(config)
+
+        assert core.timing.state.sprint_mode == False
+
+        core.toggle_sprint_mode()
+        assert core.timing.state.sprint_mode == True
+
+        core.toggle_sprint_mode()
+        assert core.timing.state.sprint_mode == False
\ No newline at end of file
diff --git a/tests/test_headless_engine.py b/tests/test_headless_engine.py
new file mode 100644
index 0000000..c1180c5
--- /dev/null
+++ b/tests/test_headless_engine.py
@@ -0,0 +1,218 @@
+"""Tests for headless simulation engine."""
+
+import time
+from unittest.mock import Mock, patch
+
+from engines.headless_engine import HeadlessSimulationEngine, HeadlessConfig
+from config.simulation_config import SimulationConfig
+
+
+class TestHeadlessConfig:
+    """Test headless configuration."""
+
+    def test_custom_values(self):
+        """Test custom configuration values."""
+        sim_config = SimulationConfig(initial_cells=50, default_tps=120.0)
+
+        config = HeadlessConfig(
+            simulation=sim_config,
+            max_ticks=10000,
+            max_duration=300.0,
+            output_dir="custom_output",
+            enable_metrics=False,
+            enable_entities=True,
+            enable_evolution=False,
+            metrics_interval=50,
+            entities_interval=500,
+            evolution_interval=2000,
+            output_formats=['json', 'csv'],
+            real_time=True
+        )
+
+        assert config.simulation == sim_config
+        assert config.max_ticks == 10000
+        assert config.max_duration == 300.0
+        assert config.output_dir == "custom_output"
+        assert config.enable_metrics == False
+        assert config.enable_entities == True
+        assert config.enable_evolution == False
+        assert config.metrics_interval == 50
+        assert config.entities_interval == 500
+        assert config.evolution_interval == 2000
+        assert config.output_formats == ['json', 'csv']
+        assert config.real_time == True
+
+
+class TestHeadlessSimulationEngine:
+    """Test headless simulation engine."""
+
+    def test_initialization(self):
+        """Test engine initialization."""
+        sim_config = SimulationConfig(initial_cells=5, initial_food=10)
+        config = HeadlessConfig(
+            simulation=sim_config,
+            max_ticks=1000,
+            output_formats=['json']
+        )
+
+        engine = HeadlessSimulationEngine(config)
+
+        assert engine.config == config
+        assert engine.event_bus is not None
+        assert engine.simulation_core is not None
+        assert engine.file_writer is not None
+        assert engine.formatters is not None
+        assert engine.collectors is not None
+        assert engine.running == False
+        assert engine.start_time is None
+
+    def test_collectors_creation(self):
+        """Test collectors are created based on configuration."""
+        sim_config = SimulationConfig()
+        config = HeadlessConfig(
+            simulation=sim_config,
+            enable_metrics=True,
+            enable_entities=True,
+            enable_evolution=False,
+            metrics_interval=50,
+            entities_interval=200
+        )
+
+        engine = HeadlessSimulationEngine(config)
+
+        assert 'metrics' in engine.collectors
+        assert 'entities' in engine.collectors
+        assert 'evolution' not in engine.collectors
+        assert engine.collectors['metrics'].collection_interval == 50
+        assert engine.collectors['entities'].collection_interval == 200
+
+    def test_formatters_creation(self):
+        """Test formatters are created based on configuration."""
+        sim_config = SimulationConfig()
+        config = HeadlessConfig(
+            simulation=sim_config,
+            output_formats=['json', 'csv']
+        )
+
+        engine = HeadlessSimulationEngine(config)
+
+        assert 'json' in engine.formatters
+        assert 'csv' in engine.formatters
+
+    def test_should_terminate_max_ticks(self):
+        """Test termination condition for max ticks."""
+        sim_config = SimulationConfig()
+        config = HeadlessConfig(simulation=sim_config, max_ticks=100)
+
+        engine = HeadlessSimulationEngine(config)
+        engine.running = True
+        engine.start_time = time.time()
+
+        # Mock the simulation core to report tick count
+        engine.simulation_core.state.total_ticks = 99
+        assert engine._should_terminate() == False
+
+        engine.simulation_core.state.total_ticks = 100
+        assert engine._should_terminate() == True
+
+    def test_should_terminate_max_duration(self):
+        """Test termination condition for max duration."""
+        sim_config = SimulationConfig()
+        config = HeadlessConfig(simulation=sim_config, max_duration=1.0)
+
+        engine = HeadlessSimulationEngine(config)
+        engine.running = True
+        engine.start_time = time.time()
+
+        # Should not terminate immediately
+        assert engine._should_terminate() == False
+
+        # Mock time passage
+        with patch('time.time', return_value=engine.start_time + 1.5):
+            assert engine._should_terminate() == True
+
+    def test_should_terminate_no_limits(self):
+        """Test no termination conditions."""
+        sim_config = SimulationConfig()
+        config = HeadlessConfig(simulation=sim_config)
+
+        engine = HeadlessSimulationEngine(config)
+        engine.running = True
+        engine.start_time = time.time()
+
+        # Should never terminate without limits
+        assert engine._should_terminate() == False
+
+    def test_collect_data(self):
+        """Test data collection from collectors."""
+        sim_config = SimulationConfig()
+        config = HeadlessConfig(
+            simulation=sim_config,
+            enable_metrics=True,
+            enable_entities=False,
+            enable_evolution=False
+        )
+
+        engine = HeadlessSimulationEngine(config)
+
+        # Mock simulation core's get_world_state method
+        engine.simulation_core.get_world_state = Mock(return_value={
+            'tick_count': 1000,
+        'actual_tps': 60.0,
+            'entity_counts': {'total': 25}
+        })
+
+        # Mock collector
+        mock_collector = Mock()
+        mock_collector.update.return_value = [
+            {'tick_count': 1000, 'actual_tps': 60.0, 'collection_type': 'metrics'}
+        ]
+        engine.collectors['metrics'] = mock_collector
+
+        engine._collect_data()
+
+        # Verify collector was called
+        mock_collector.update.assert_called_once()
+
+        # Verify data was collected
+        assert len(engine.batch_data['metrics']) == 1
+        assert engine.batch_data['metrics'][0]['tick_count'] == 1000
+
+    def test_get_real_time_status(self):
+        """Test real-time status reporting."""
+        sim_config = SimulationConfig()
+        config = HeadlessConfig(simulation=sim_config)
+
+        engine = HeadlessSimulationEngine(config)
+        engine.running = True
+        engine.start_time = time.time() - 5.0
+
+        # Mock simulation state
+        engine.simulation_core.state.total_ticks = 300
+        engine.simulation_core.state.actual_tps = 60.0
+        engine.simulation_core.get_world_state = Mock(return_value={
+            'tick_count': 300,
+            'actual_tps': 60.0,
+            'entity_counts': {'total': 50}
+        })
+
+        status = engine.get_real_time_status()
+
+        assert status['running'] == True
+        assert status['ticks'] == 300
+        assert status['tps'] == 60.0
+        assert status['duration'] > 4.0  # Approximately 5 seconds
+        assert status['world_state']['tick_count'] == 300
+
+    def test_signal_handler(self):
+        """Test signal handling for graceful shutdown."""
+        sim_config = SimulationConfig()
+        config = HeadlessConfig(simulation=sim_config)
+
+        engine = HeadlessSimulationEngine(config)
+        engine.running = True
+
+        # Simulate signal handler
+        engine._signal_handler(2, None)  # SIGINT
+
+        assert engine.running == False
\ No newline at end of file
diff --git a/tests/test_output_system.py b/tests/test_output_system.py
new file mode 100644
index 0000000..950b4b9
--- /dev/null
+++ b/tests/test_output_system.py
@@ -0,0 +1,195 @@
+"""Tests for output collection system."""
+
+import json
+import tempfile
+import os
+from unittest.mock import Mock
+
+from output.collectors.metrics_collector import MetricsCollector
+from output.collectors.entity_collector import EntityCollector
+from output.formatters.json_formatter import JSONFormatter
+from output.formatters.csv_formatter import CSVFormatter
+from output.writers.file_writer import FileWriter
+
+
+class TestMetricsCollector:
+    """Test metrics collector functionality."""
+
+    def test_collect_data(self):
+        """Test metrics data collection."""
+        collector = MetricsCollector(collection_interval=100)
+
+        # Create mock simulation core
+        mock_sim_core = Mock()
+        mock_sim_core.get_world_state.return_value = {
+            'tick_count': 1500,
+            'actual_tps': 58.5,
+            'target_tps': 60.0,
+            'speed_multiplier': 1.0,
+            'is_paused': False,
+            'sprint_mode': False,
+            'world_buffer': 1,
+            'entity_counts': {
+                'total': 25,
+                'cells': 20,
+                'food': 5
+            }
+        }
+
+        # Mock timing
+        mock_sim_core.timing = Mock()
+        mock_sim_core.timing.last_tick_time = 1234567890.5
+
+        data = collector.collect(mock_sim_core)
+
+        assert data['tick_count'] == 1500
+        assert data['actual_tps'] == 58.5
+        assert data['target_tps'] == 60.0
+        assert data['speed_multiplier'] == 1.0
+        assert data['is_paused'] == False
+        assert data['sprint_mode'] == False
+        assert data['world_buffer'] == 1
+        assert data['entity_counts'] == {
+            'total': 25,
+            'cells': 20,
+            'food': 5
+        }
+        assert data['collection_type'] == 'metrics'
+        assert data['timestamp'] == 1234567890.5
+
+
+class TestEntityCollector:
+    """Test entity collector functionality."""
+
+    def test_collect_cells_only(self):
+        """Test collecting only cell entities."""
+        collector = EntityCollector(
+            collection_interval=1000,
+            include_cells=True,
+            include_food=False
+        )
+
+        # Create mock simulation core with entity states
+        mock_sim_core = Mock()
+        mock_sim_core.get_world_state.return_value = {
+            'tick_count': 1000
+        }
+        mock_sim_core.get_entity_states.return_value = [
+            {
+                'id': 1,
+                'type': 'cell',
+                'position': {'x': 10, 'y': 20},
+                'energy': 75.5,
+                'age': 150,
+                'generation': 3,
+                'neural_network': {'layer_sizes': [4, 6, 2]}
+            },
+            {
+                'id': 2,
+                'type': 'food',
+                'position': {'x': 30, 'y': 40},
+                'decay': 50,
+                'max_decay': 100
+            }
+        ]
+
+        data = collector.collect(mock_sim_core)
+
+        assert len(data['entities']) == 1  # Only cell included
+        assert data['entities'][0]['type'] == 'cell'
+        assert data['entities'][0]['id'] == 1
+        assert data['collection_type'] == 'entities'
+
+    
+
+class TestJSONFormatter:
+    """Test JSON formatter functionality."""
+
+    def test_format_data(self):
+        """Test JSON data formatting."""
+        formatter = JSONFormatter()
+
+        test_data = {
+            'tick_count': 1000,
+            'actual_tps': 58.5,
+            'entity_counts': {
+                'cells': 20,
+                'food': 5
+            }
+        }
+
+        formatted = formatter.format(test_data)
+
+        assert isinstance(formatted, str)
+
+        # Verify it's valid JSON
+        parsed = json.loads(formatted)
+        assert parsed['tick_count'] == 1000
+        assert parsed['actual_tps'] == 58.5
+        assert parsed['entity_counts']['cells'] == 20
+
+
+class TestCSVFormatter:
+    """Test CSV formatter functionality."""
+
+    def test_format_simple_data(self):
+        """Test CSV formatting for simple data."""
+        formatter = CSVFormatter()
+
+        test_data = {
+            'tick_count': 1000,
+            'actual_tps': 58.5,
+            'is_paused': False
+        }
+
+        formatted = formatter.format(test_data)
+
+        assert isinstance(formatted, str)
+        lines = formatted.strip().split('\n')
+
+        # Should have header and one data row
+        assert len(lines) == 2
+
+        # Check header
+        header = lines[0]
+        assert 'tick_count' in header
+        assert 'actual_tps' in header
+        assert 'is_paused' in header
+
+        # Check data row
+        data_row = lines[1]
+        assert '1000' in data_row
+        assert '58.5' in data_row
+
+    def test_get_file_extension(self):
+        """Test file extension."""
+        formatter = CSVFormatter()
+        assert formatter.get_file_extension() == 'csv'
+
+
+class TestFileWriter:
+    """Test file writer functionality."""
+
+    def test_write_data(self):
+        """Test writing data to file."""
+        # Create temporary directory
+        with tempfile.TemporaryDirectory() as temp_dir:
+            writer = FileWriter(temp_dir)
+
+            # Should be ready now
+            assert writer.is_ready() == True
+
+            # Write test data
+            test_data = '{"tick_count": 1000, "actual_tps": 58.5}'
+            filename = "test_data.json"
+
+            writer.write(test_data, filename)
+
+            # Verify file was created
+            file_path = os.path.join(temp_dir, filename)
+            assert os.path.exists(file_path)
+
+            # Verify file content
+            with open(file_path, 'r') as f:
+                content = f.read()
+                assert content == test_data
\ No newline at end of file