Abstract: An anti-theft alarm is provided, for an externally powered electrically operated device which has an openable case containing internal components. The alarm comprises:a) a rechargeable batteryb) a motion sensor to detect movement of the device, and a tamper switch to determine if the case is about to be opened or has been opened,c) a DC power supply connected to a circuit for recharging the battery, the DC supply being dependent on the external power supply for the device,d) a warning means activated by controller, powered by the battery, the controller being activated by a condition selected from the group consisting of i) no DC power and activation of the motion sensor and ii) activation of the tamper switche) an arming latch which keeps the warning means activated when either of the conditions in d) is satisfiedf) a key switch which is adapted to disarm the alarm when the DC power is off.The alarm is particularly useful for computers which are valuable in themselves or have valuable components.