This article follows the development of surgical diathermy equipment from the early part of this century to the present day. The principles of operation of both valve/spark gap and solid state units are given and some of the hazards associated with the use of diathermy discussed. The two main types of output circuit, the rf earth referred and the fully isolated are discussed and their relative merits and failings compared. Some advice is also offered on the selection and purchase of diathermy equipment.