Abstract: An identification system comprises an interrogator and a plurality of transponders. The interrogator includes a transmitter which transmits at least two independent interrogation signals to the transponders, and a receiver for receiving response signals from the transponders. The interrogator also includes processor means for identifying the transponders from data in the received response signals. Each transponder comprises receiving means, a code generator, and transmitter means connected to the code generator. On receipt of at least one of the transmitted interrogation signals the transponder transmits a response signal containing data which identifies it. The interrogation signals may be transmitted continuously or intermittently. In a preferred embodiment, the interrogation signals are relatively narrow bandwidth signals at different frequencies.