The considerations leading to the development of secondary surveillance radar (s.s.r.) for air-traffic control are listed. The evolution of s.s.r. is described from a simple responder capable of operation with primary radar to a complex system employing special frequencies and coding facilities on the ground-to-air and air-to-ground channels. The evaluation at London Airport of a relatively simple independent s.s.r. system in 1954 and 1956 and subsequent international discussions on the subject led to standardisation of system characteristics in 1957 by the International Civil Aviation Organisation (ICAO). Development of a British system designed to satisfy all the ICAO requirements was finished at the end of 1960 and was evaluated at London Airport during 1961. It was concluded that this system could reliably form the basis of an air-traffic-control radar recognition system. An experimental height-transmission equipment employing the air-to-ground coding system is described, and requirements for future system development are outlined.