Abstract: An engine security system includes a security device adapted to receive a sequence of timing pulses which is generated in dependence on the rotation of the engine. The device generates an ignition pulse sequence synchronous with rotation of the engine from the sequence of timing pulses on receipt of an enabling code. The ignition pulse sequence is supplied to the ignition system of the engine for the generation of ignition sparks. The pulses in the timing pulse sequence occur at different times from pulses in the ignition pulse sequence. In preferred embodiments, the sequence of timing pulses includes an identity code encoded therein.