MODERN turbojet engines have a shaft speed range of approximately 2 to 1 (from ground idle to take‐off) and it is necessary, therefore, to have some form of infinitely‐variable‐ratio gearbox to obtain a constant speed for accessories such as a.c. generators. Various means of doing this are available; for example, these could be mechanical, electrical, hydraulic or pneumatic, or a combination of these. The purpose of this paper is to discuss the major types of constant speed drive (C.S.D.) that have been evolved, mentioning the design problems associated with each.