Abstract: An object identification system comprises a transceiver (1) and a transponder (30) including memories (33,34) for storing an identification code identifying the transponder. Circuits (35, 36, 37, 38) generate an identification signal representative of the identification code such that the identification signal is modulated by bits of the identification code. Circuits (8, 9, 10, 11, 12) of the transceiver receive and interpret the identification signal. The transponder includes a variable code generator (32) and a controller (31) for controlling the writing of the variable code into memory (33) so that the variable code constitutes part of the identification code. The variable code may be a random or pseudo-random code.