Abstract: A fuel theft detection system (50) for detecting when fuel has been stolen from a vehicle's fuel tank (16) is disclosed. The detection system (50) comprises input means (56) that is connectable to a vehicle's fuel level sensor (14) for providing an indication of the fuel level in the vehicle's fuel tank (16). A controller (22) is connected to the input means (56) for controlling the system (50), with the system (50) further including timing means (72) for driving the controller (22) to capture and record the fuel level at regular, pre-determined intervals. The controller (22) is arranged to calculate and store the average fuel level over successive periods of time, with any deviation in the fuel level by more than a predetermined amount suggesting fuel theft. Significantly, the controller (22) is simultaneously able to determine fuel theft when fuel is being dispensed into the fuel tank (16).