Abstract: To prevent unintentional alarm initiation in an intruder detection system of the kind in which a transmitter emits radio waves across an area to be monitored and a receiver senses changes in the resultant signal, which initiation might otherwise occur as a result of environmental changes or natural causes such as the entry of a small animal into the monitored area, a receiver for such a system is adapted to sense the received signal at predetermined intervals of time and only to cause alarm initiation in the event that the change occurring in any one of said predetermined intervals exceeds a predetermined amount. Such an arrangement serves, therefore, to compensate automatically for changes, in the received signal, arising from environmental changes.