A method of identifying a plurality of transponders, the method comprising receiving at an interrogator one or more data transmission blocks from each transponder, wherein the first data block contains a uniquely detectable transponder identity and zero or more further data blocks defining a set of data blocks making up a transponder transmission. The first data transmission block also contains in transmission block number; one or more further data transmission blocks containing additional block data and a respective transmission block number; the block number being changed, by decrement or increment, automatically as each data block is transmitted.