Abstract: An intruder detection system comprising a pair of detection nodes supporting a bidirectional wireless communication link across which each node is capable of sending a wireless signal to and receiving a wireless signal from the other paired node. Each node analyses the wireless signal received from the other node to detect one or more characteristics of the received signal that is indicative of a target in the detection zone. Each node is operable in a transmit mode in which it transmits to the other paired node, and a receive mode in which it receives from said other paired node, each node switching periodically between modes.