The discovery of the Gunn Effect has been heralded as the greatest breakthrough in semiconductor physics since the discovery in 1948 of transistor action in germanium. This article shows the basic physics of the effect and lays emphasis on the physics and applications of devices in which there is a bulk negative differential conductivity. Exciting new possibilities are opened up in the microwave region by a range of devices based on the peculiar properties of high field domains in semiconductors like GaAs.