Alarm system for motor vehicle, incorporating an alarm control unit which is powered by a main battery of the motor vehicle, a siren unit likewise powered by the main battery, and a back-up battery in the siren unit which is arranged to power the siren unit upon failure of the power supply from the main battery. To simplify and make such an alarm safer, it is proposed inter alia that upon said failure of power supply from the main battery the back-up battery be arranged to automatically power also the alarm control unit.