After a survey of the events and developments which led to the start of the BBC's high-definition television service, this monograph is a narration of the main engineering developments during the twenty-five years since the service started. Particular attention is paid to those techniques which have been originated by the BBC or which are utilized in a specially noteworthy manner in the BBC service. There is also a brief review of the BBC's contribution to probable future developments including colour television and a change of standards.