High-definition television was described in 1935 by the Selsdon Committee as having not less than 240 lines per picture. Public transmission of an interlaced 405-line service commenced in 1936 and lasted until 1985, having been gradually displaced by 625-line colour. Today, high definition means more than 1000 lines, and is unlikely to be broadcast from conventional terrestrial transmitters.This Address reviews the development of terrestrial television transmission technology from the, pre-war Baird/Marconi-EMI trials, through the period of post-war Band I and III expansion to the introduction of u.h.f. and the ultimate realization in the United Kingdom of one of the most highly developed and effective colour transmission networks anywhere in the world.