Abstract: A vehicle theft prevention device (10) arranged to be fitted to the air inlet/outlet of a vehicle tire includes a motion detector (11) and an air release means (12). The device (10) can be armed so that, in use, a solenoid controlled valve (13) in the air release means (12) is arranged to be opened when the motion detector (11) senses movement. If movement is detected air held under pressure in the tire is allowed to escape and the vehicle is disabled. The device (10) may be disarmed so that an authorized user may use the vehicle without unnecessary deflation of one or more tires of the vehicle. The device (10) also includes a standard maintenance valve (14) for adjusting the air pressure of a tire in normal use.