Abstract: A vehicle anti-theft alarm system comprises a microwave transmitter and receiver for transmitting microwave frequency signals from one location in the vehicle against a target in the vehicle at a given distance from the transmitter-receiver and for receiving the signals reflected back from the target and producing a monitoring signal representative of a time, frequency or distance parameter for sending and receiving the signals. If, for example, an object is interposed in the path of the transmitted signals, the parameter and thus the monitoring signal changes and a control signal is produced for actuating an alarm. A holding circuit provides for the alarm to remain actuated when the article is removed from the path of the transmitted signals.