diff --git a/CommonLib/SocketHelper.py b/CommonLib/SocketHelper.py
index 590c34fe..7cfd5958 100644
--- a/CommonLib/SocketHelper.py
+++ b/CommonLib/SocketHelper.py
@@ -150,7 +150,7 @@ def sendData(self, simState, simTime, sock):
         byte_index = byte_index + self.msg_header_size
         vehicle_byte_index = 0
         vehicle_data_buffer = bytearray(65536) 
-        # TODO: add traffic light and detector data
+        # Not yet implemented - see GitHub issue #130
         traffic_light_data_buffer = bytearray(8096)
         detector_data_buffer = bytearray(8096)
         data_buffer = bytearray(81728)
diff --git a/CommonLib/__init__.py b/CommonLib/__init__.py
new file mode 100644
index 00000000..72b7354f
--- /dev/null
+++ b/CommonLib/__init__.py
@@ -0,0 +1,4 @@
+from .ConfigHelper import ConfigHelper
+from .MsgHelper import MsgHelper
+from .SocketHelper import SocketHelper
+from .VehDataMsgDefs import VehData
diff --git a/environment.yml b/environment.yml
index 15253bc0..584b429a 100644
--- a/environment.yml
+++ b/environment.yml
@@ -11,8 +11,10 @@ dependencies:
   - plotly
   - pygame
   - protobuf
+  - pytest
+  - pyyaml
   - vs2015_runtime
   - vc14_runtime
   - pip
   - pip:
-      - ./Carla/carla-0.9.15-cp310-cp310-win_amd64.whl
\ No newline at end of file
+      - ./Carla/carla-0.9.15-cp310-cp310-win_amd64.whl
diff --git a/requirements.txt b/requirements.txt
index 55bd938f..3e4402f4 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,14 +1,9 @@
-# External software (not installed via pip)
-# vissim==11.0
-# sumo==1.13.0
-# carmaker==10.0.1
-# matlab==9.7
-
 # Python packages
 easydict>=1.9
 matplotlib>=3.5.0
 numpy>=1.23.0
 pandas>=1.5.0
+pytest>=7.0.0
 pyyaml>=6.0
 ruamel.yaml>=0.17.0
 sphinx>=5.0.0
diff --git a/tests/Python/SimpleEchoClient/simple_echo_client.py b/tests/Python/SimpleEchoClient/simple_echo_client.py
index 02e29965..457d4d91 100644
--- a/tests/Python/SimpleEchoClient/simple_echo_client.py
+++ b/tests/Python/SimpleEchoClient/simple_echo_client.py
@@ -6,9 +6,10 @@
 import socket
 import sys
 import os
+import pathlib
 
-# Add parent directory to path to import CommonLib
-sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__), '..', '..', '..')))
+# Add repo root to path to import CommonLib
+sys.path.insert(0, str(pathlib.Path(__file__).parents[3]))
 
 from CommonLib.SocketHelper import SocketHelper
 from CommonLib.MsgHelper import MsgHelper
diff --git a/tests/Python/SimpleTrafficLight/echo_client.py b/tests/Python/SimpleTrafficLight/echo_client.py
index 55f2adbd..0b589d16 100644
--- a/tests/Python/SimpleTrafficLight/echo_client.py
+++ b/tests/Python/SimpleTrafficLight/echo_client.py
@@ -6,9 +6,10 @@
 import socket
 import sys
 import os
+import pathlib
 
-# Add CommonLib to path
-repo_root = os.path.abspath(os.path.join(os.path.dirname(__file__), '..', '..', '..'))
+# Add repo root to path to import CommonLib
+repo_root = str(pathlib.Path(__file__).parents[3])
 if repo_root not in sys.path:
     sys.path.insert(0, repo_root)
 
diff --git a/tests/Python/conftest.py b/tests/Python/conftest.py
new file mode 100644
index 00000000..ca09f184
--- /dev/null
+++ b/tests/Python/conftest.py
@@ -0,0 +1,4 @@
+import sys
+import pathlib
+# Add repo root to path for all tests
+sys.path.insert(0, str(pathlib.Path(__file__).parents[2]))
diff --git a/tests/Python/pytest.ini b/tests/Python/pytest.ini
new file mode 100644
index 00000000..0bc104e4
--- /dev/null
+++ b/tests/Python/pytest.ini
@@ -0,0 +1,5 @@
+[pytest]
+testpaths = .
+python_files = test_*.py
+python_classes = Test*
+python_functions = test_*
diff --git a/tests/Python/unit/__init__.py b/tests/Python/unit/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/tests/Python/unit/test_config_helper.py b/tests/Python/unit/test_config_helper.py
new file mode 100644
index 00000000..c6be8c86
--- /dev/null
+++ b/tests/Python/unit/test_config_helper.py
@@ -0,0 +1,85 @@
+import pytest
+import os
+from CommonLib.ConfigHelper import ConfigHelper
+
+SIMPLE_ECHO_CONFIG = os.path.join(
+    os.path.dirname(__file__), '..', 'SimpleEchoClient', 'config.yaml'
+)
+
+
+def test_load_simple_echo_config():
+    """ConfigHelper can load the SimpleEchoClient config without errors."""
+    if not os.path.exists(SIMPLE_ECHO_CONFIG):
+        pytest.skip("config file not found")
+    ch = ConfigHelper()
+    ch.getConfig(SIMPLE_ECHO_CONFIG)
+    assert ch.simulation_setup is not None
+
+
+def test_config_simulation_setup_keys():
+    """Required SimulationSetup fields are populated after loading config."""
+    if not os.path.exists(SIMPLE_ECHO_CONFIG):
+        pytest.skip("config file not found")
+    ch = ConfigHelper()
+    ch.getConfig(SIMPLE_ECHO_CONFIG)
+
+    assert ch.simulation_setup['SelectedTrafficSimulator'] == 'SUMO'
+    assert ch.simulation_setup['TrafficSimulatorIP'] == '127.0.0.1'
+    assert isinstance(ch.simulation_setup['TrafficSimulatorPort'], int)
+    assert ch.simulation_setup['EnableRealSim'] is True
+    assert ch.simulation_setup['EnableExternalDynamics'] is True
+
+
+def test_config_vehicle_message_field():
+    """VehicleMessageField is a non-empty list of strings."""
+    if not os.path.exists(SIMPLE_ECHO_CONFIG):
+        pytest.skip("config file not found")
+    ch = ConfigHelper()
+    ch.getConfig(SIMPLE_ECHO_CONFIG)
+
+    fields = ch.simulation_setup['VehicleMessageField']
+    assert isinstance(fields, list)
+    assert len(fields) > 0
+    for f in fields:
+        assert isinstance(f, str)
+
+
+def test_config_application_setup():
+    """ApplicationSetup is populated with VehicleSubscription list."""
+    if not os.path.exists(SIMPLE_ECHO_CONFIG):
+        pytest.skip("config file not found")
+    ch = ConfigHelper()
+    ch.getConfig(SIMPLE_ECHO_CONFIG)
+
+    assert ch.application_setup['EnableApplicationLayer'] is True
+    subs = ch.application_setup['VehicleSubscription']
+    assert isinstance(subs, list)
+    assert len(subs) > 0
+    first = subs[0]
+    assert 'type' in first
+    assert 'ip' in first
+    assert 'port' in first
+
+
+def test_config_defaults_for_missing_fields():
+    """ConfigHelper returns sensible defaults for fields not present in yaml."""
+    if not os.path.exists(SIMPLE_ECHO_CONFIG):
+        pytest.skip("config file not found")
+    ch = ConfigHelper()
+    ch.getConfig(SIMPLE_ECHO_CONFIG)
+
+    # SimulationMode not in config — should default to 0
+    assert ch.simulation_setup['SimulationMode'] == 0
+    # SimulationEndTime not in config — should default to 90000
+    assert ch.simulation_setup['SimulationEndTime'] == 90000.0
+
+
+def test_reset_config():
+    """resetConfig clears all setup dicts."""
+    if not os.path.exists(SIMPLE_ECHO_CONFIG):
+        pytest.skip("config file not found")
+    ch = ConfigHelper()
+    ch.getConfig(SIMPLE_ECHO_CONFIG)
+    ch.resetConfig()
+    assert len(ch.simulation_setup) == 0
+    assert len(ch.application_setup) == 0
diff --git a/tests/Python/unit/test_msg_helper.py b/tests/Python/unit/test_msg_helper.py
new file mode 100644
index 00000000..e595c813
--- /dev/null
+++ b/tests/Python/unit/test_msg_helper.py
@@ -0,0 +1,172 @@
+import struct
+import pytest
+from CommonLib.MsgHelper import MsgHelper, MessageType
+from CommonLib.VehDataMsgDefs import VehData
+
+
+def make_msg_helper_with_fields(*fields):
+    """Return a MsgHelper with the given vehicle fields enabled."""
+    mh = MsgHelper()
+    mh.set_vehicle_message_field(list(fields))
+    return mh
+
+
+class TestMsgHelperInit:
+    def test_default_fields_all_false(self):
+        mh = MsgHelper()
+        for key, val in mh.vehicle_msg_field_valid.items():
+            assert val is False, f"Expected {key} to be False by default"
+
+    def test_header_sizes(self):
+        mh = MsgHelper()
+        assert mh.msg_header_size == 9
+        assert mh.msg_each_header_size == 3
+
+
+class TestSetVehicleMessageField:
+    def test_set_fields_marks_valid(self):
+        mh = MsgHelper()
+        mh.set_vehicle_message_field(['id', 'speed', 'positionX'])
+        assert mh.vehicle_msg_field_valid['id'] is True
+        assert mh.vehicle_msg_field_valid['speed'] is True
+        assert mh.vehicle_msg_field_valid['positionX'] is True
+
+    def test_unset_fields_remain_false(self):
+        mh = MsgHelper()
+        mh.set_vehicle_message_field(['id', 'speed'])
+        assert mh.vehicle_msg_field_valid['positionY'] is False
+        assert mh.vehicle_msg_field_valid['heading'] is False
+
+
+class TestPackUnpackString:
+    def test_roundtrip_short_string(self):
+        buf = bytearray(64)
+        packed, idx = MsgHelper.pack_string(buf, 0, "hello")
+        assert idx == 6  # 1 byte length + 5 bytes string
+        result, length, new_idx, arr = MsgHelper.depack_string(packed, 0)
+        assert result == "hello"
+        assert length == 5
+        assert new_idx == 6
+
+    def test_roundtrip_empty_string(self):
+        buf = bytearray(16)
+        packed, idx = MsgHelper.pack_string(buf, 0, "")
+        assert idx == 1
+        result, length, new_idx, arr = MsgHelper.depack_string(packed, 0)
+        assert result == ""
+        assert length == 0
+
+    def test_roundtrip_with_offset(self):
+        buf = bytearray(64)
+        # Put some bytes at start then pack string at offset 4
+        MsgHelper.pack_string(buf, 4, "abc")
+        result, length, _, _ = MsgHelper.depack_string(buf, 4)
+        assert result == "abc"
+        assert length == 3
+
+
+class TestUnpackHelpers:
+    def test_unpack_float(self):
+        raw = struct.pack('f', 3.14)
+        val, idx = MsgHelper.unpack_float(raw, 0)
+        assert abs(val - 3.14) < 0.001
+        assert idx == 4
+
+    def test_unpack_int32(self):
+        raw = struct.pack('i', -42)
+        val, idx = MsgHelper.unpack_int32(raw, 0)
+        assert val == -42
+        assert idx == 4
+
+    def test_unpack_uint32(self):
+        raw = struct.pack('I', 999999)
+        val, idx = MsgHelper.unpack_uint32(raw, 0)
+        assert val == 999999
+
+    def test_unpack_uint16(self):
+        raw = struct.pack('H', 1234)
+        val, idx = MsgHelper.unpack_uint16(raw, 0)
+        assert val == 1234
+        assert idx == 2
+
+    def test_unpack_uint8(self):
+        raw = bytes([255])
+        val, idx = MsgHelper.unpack_uint8(raw, 0)
+        assert val == 255
+        assert idx == 1
+
+    def test_unpack_int8(self):
+        raw = struct.pack('b', -1)
+        val, idx = MsgHelper.unpack_int8(raw, 0)
+        assert val == -1
+        assert idx == 1
+
+
+class TestPackDepackVehData:
+    def test_roundtrip_id_and_speed(self):
+        mh = make_msg_helper_with_fields('id', 'speed')
+        veh = VehData()
+        veh.id = 'car1'
+        veh.speed = 15.5
+
+        buf = bytearray(256)
+        packed, msg_size, end_idx = mh.pack_veh_data(buf, 0, veh)
+
+        # depack_veh_data skips the 3-byte sub-header
+        unpacked = mh.depack_veh_data(bytes(packed[mh.msg_each_header_size:end_idx]))
+        assert unpacked.id == 'car1'
+        assert abs(unpacked.speed - 15.5) < 0.001
+
+    def test_roundtrip_position_fields(self):
+        mh = make_msg_helper_with_fields('positionX', 'positionY', 'positionZ', 'heading')
+        veh = VehData()
+        veh.positionX = 100.0
+        veh.positionY = 200.0
+        veh.positionZ = 5.0
+        veh.heading = 1.57
+
+        buf = bytearray(256)
+        packed, msg_size, end_idx = mh.pack_veh_data(buf, 0, veh)
+        unpacked = mh.depack_veh_data(bytes(packed[mh.msg_each_header_size:end_idx]))
+
+        assert abs(unpacked.positionX - 100.0) < 0.01
+        assert abs(unpacked.positionY - 200.0) < 0.01
+        assert abs(unpacked.positionZ - 5.0) < 0.01
+        assert abs(unpacked.heading - 1.57) < 0.01
+
+    def test_roundtrip_multiple_string_fields(self):
+        mh = make_msg_helper_with_fields('id', 'type', 'vehicleClass')
+        veh = VehData()
+        veh.id = 'ego'
+        veh.type = 'car'
+        veh.vehicleClass = 'passenger'
+
+        buf = bytearray(256)
+        packed, msg_size, end_idx = mh.pack_veh_data(buf, 0, veh)
+        unpacked = mh.depack_veh_data(bytes(packed[mh.msg_each_header_size:end_idx]))
+
+        assert unpacked.id == 'ego'
+        assert unpacked.type == 'car'
+        assert unpacked.vehicleClass == 'passenger'
+
+    def test_message_type_byte_is_vehicle_data(self):
+        mh = make_msg_helper_with_fields('id')
+        veh = VehData()
+        veh.id = 'v1'
+
+        buf = bytearray(256)
+        packed, _, _ = mh.pack_veh_data(buf, 0, veh)
+
+        # byte 2 of sub-header is the message type
+        assert packed[2] == MessageType.vehicle_data
+
+    def test_pack_header(self):
+        mh = MsgHelper()
+        buf = bytearray(256)
+        packed, idx = mh.pack_msg_header(buf, 1, 5.5, 100)
+        assert idx == mh.msg_header_size
+
+        sim_state, sim_time, total = mh.depack_msg_header(bytes(buf[:mh.msg_header_size]))
+        assert sim_state == 1
+        assert abs(sim_time - 5.5) < 0.001
+        assert total == 100
diff --git a/tests/Python/unit/test_veh_data.py b/tests/Python/unit/test_veh_data.py
new file mode 100644
index 00000000..0ca80d16
--- /dev/null
+++ b/tests/Python/unit/test_veh_data.py
@@ -0,0 +1,103 @@
+import pytest
+from CommonLib.VehDataMsgDefs import VehData, TrafficLightData, DetectorData
+
+
+class TestVehData:
+    def test_instantiation_no_args(self):
+        """VehData can be created with no arguments (all fields have defaults)."""
+        veh = VehData()
+        assert veh is not None
+
+    def test_default_numeric_fields(self):
+        """Numeric fields default to zero."""
+        veh = VehData()
+        assert veh.speed == 0.0
+        assert veh.acceleration == 0.0
+        assert veh.positionX == 0.0
+        assert veh.positionY == 0.0
+        assert veh.positionZ == 0.0
+        assert veh.heading == 0.0
+        assert veh.color == 0
+        assert veh.laneId == 0
+        assert veh.hasPrecedingVehicle == 0
+        assert veh.activeLaneChange == 0
+
+    def test_default_string_fields_exist(self):
+        """String fields (id, type, etc.) exist and are strings."""
+        veh = VehData()
+        assert isinstance(veh.id, str)
+        assert isinstance(veh.type, str)
+        assert isinstance(veh.vehicleClass, str)
+        assert isinstance(veh.linkId, str)
+        assert isinstance(veh.precedingVehicleId, str)
+        assert isinstance(veh.signalLightId, str)
+        assert isinstance(veh.linkIdNext, str)
+
+    def test_expected_fields_present(self):
+        """All expected fields are accessible."""
+        veh = VehData()
+        expected_fields = [
+            'id', 'type', 'vehicleClass', 'speed', 'acceleration',
+            'positionX', 'positionY', 'positionZ', 'heading', 'color',
+            'linkId', 'laneId', 'distanceTravel', 'speedDesired',
+            'accelerationDesired', 'hasPrecedingVehicle', 'precedingVehicleId',
+            'precedingVehicleDistance', 'precedingVehicleSpeed',
+            'signalLightId', 'signalLightHeadId', 'signalLightDistance',
+            'signalLightColor', 'speedLimit', 'speedLimitNext',
+            'speedLimitChangeDistance', 'linkIdNext', 'grade',
+            'length', 'width', 'height', 'activeLaneChange',
+        ]
+        for f in expected_fields:
+            assert hasattr(veh, f), f"VehData missing expected field: {f}"
+
+    def test_field_assignment(self):
+        """Fields can be assigned and read back."""
+        veh = VehData()
+        veh.id = 'ego'
+        veh.speed = 27.8
+        veh.positionX = 100.0
+        veh.positionY = -50.5
+        veh.heading = 3.14
+
+        assert veh.id == 'ego'
+        assert veh.speed == 27.8
+        assert veh.positionX == 100.0
+        assert veh.positionY == -50.5
+        assert veh.heading == 3.14
+
+    def test_get_method(self):
+        """The .get() helper returns field values and defaults for missing keys."""
+        veh = VehData()
+        veh.speed = 10.0
+        assert veh.get('speed') == 10.0
+        assert veh.get('nonexistent_field', 99) == 99
+
+    def test_independent_default_strings(self):
+        """Two VehData instances do not share the same string default objects."""
+        veh1 = VehData()
+        veh2 = VehData()
+        veh1.id = 'vehicle_a'
+        assert veh2.id != 'vehicle_a'
+
+
+class TestTrafficLightData:
+    def test_instantiation_with_args(self):
+        """TrafficLightData requires positional args (no defaults)."""
+        tl = TrafficLightData(id=1, name='TL_north', state='green')
+        assert tl.id == 1
+        assert tl.name == 'TL_north'
+        assert tl.state == 'green'
+
+    def test_get_method(self):
+        tl = TrafficLightData(id=2, name='TL_south', state='red')
+        assert tl.get('state') == 'red'
+        assert tl.get('missing', 'default') == 'default'
+
+
+class TestDetectorData:
+    def test_instantiation_with_args(self):
+        """DetectorData requires positional args (no defaults)."""
+        det = DetectorData(id=5, name='det_01', state=1)
+        assert det.id == 5
+        assert det.name == 'det_01'
+        assert det.state == 1