An immobiliser or immobilizer is an electronic security device fitted to a motor vehicle that prevents the engine from being started unless the correct key (transponder or smart key) is present. This prevents the vehicle from being "hot wired" after entry has been achieved and thus reduces motor vehicle theft. Research shows that the uniform application of immobilisers reduced the rate of car theft by 40%.

The electric immobiliser/alarm system was invented by St. George Evans and Edward Birkenbeuel and patented in 1919. They developed a 3x3 grid of double-contact switches on a panel mounted inside the car so when the ignition switch was activated, current from the battery (or magneto) went to the spark plugs allowing the engine to start, or immobilizing the vehicle and sounding the horn. The system settings could be changed each time the car was driven. Modern immobiliser systems are automatic, meaning the owner does not have to remember to activate it.

Early models used a static code in the ignition key (or key fob) which was recognised by an RFID loop (transponder) around the lock barrel and checked against the vehicle's engine control unit (ECU) for a match. If the code is unrecognised, the ECU will not allow fuel to flow and ignition to take place.

Later models use rolling codes or advanced cryptography to defeat copying of the code from the key or ECU (smart key).

The microcircuit inside the key is activated by a small electromagnetic field which induces current to flow inside the key body, which in turn broadcasts a unique binary code, which is read by the automobile's ECU. When the ECU determines that the coded key is both current and valid, the ECU activates the fuel-injection sequence.

In some vehicles, attempts to use an unauthorised or "non-sequenced" key cause the vehicle to activate a timed "no-start condition" and in some highly advanced systems, even use satellite or mobile phone communication to alert a security firm that an unauthorised attempt was made to code a key.

Coincidentally, this information is often recorded in modern automobile ECUs as part of their on-board diagnostics which may record many other variables including speed, temperature, driver weight, geographic location, throttle position and yaw angle. This information can be used during insurance investigations, warranty claims or technical troubleshooting.

Immobilisers have been mandatory in all new cars sold in Germany since 1 January 1998, in the United Kingdom since 1 October 1998, in Finland since 1998, and in Australia since 2001.^{[citation needed]}