This Address gives a perspective to the field of engineering to which Dr. John Ainsworth, who has just received the 1994 IEE Power Division Achievement Award, has made a substantial contribution. Reflecting the efforts of a few of its senior engineers with whom the author has worked during the last 30 years, this Address traces the developments of an industry which, though global in application, is only a niche market in size. It briefly reviews the state of the art and provides some pointers for the future.