Air transport is characterized by a rapid and fairly steady growth in the size, numbers and speed of the vehicles. The systems for navigation and control of this traffic have evolved with the changing problem, and heavy demands have been, and are being, made on electronic technology. At the same time, heavy emphasis is laid on the need for evolutionary changes; old and new systems must be compatible. The present investment, world-wide, in civil air transport is about £100000M. World-wide compatibility is essential between air and ground electronic subsystems, and such systems and operating procedures are, eventually, the subject of international agreements. It follows that any system changes must be heavily influenced by past history, and for that reason it is proposed to discuss airborne secondary radar transponders on a historical basis.