The rapid growth of both industrial and domestic load in most countries since 1939 has forced the development of many water-power schemes, some of which had previously been dormant owing to marginal economies. Earlier schemes were often isolated and (with certain notable exceptions) of relative insignificance in comparison with steam-driven generating plant. With the present need for developing every practical source of power, however, the problems of controlling hydro-electric plant take on new aspects, involving consideration of uniformity and economy of operation and maintenance staffs, and the most efficient use of the catchments on each river system as a whole.After the terms used have been denned, the nature of the control problems involved are examined. Certain control schemes are described, with emphasis laid on the development of sequential-parallel automatic control and the reasons why it is preferred. Some practical aspects of the apparatus involved are described, after which the operating and maintenance requirements are examined. The electrical and hydraulic functions of group control are discussed in relation to complete river system. Finally, those factors influencing the choice of a control scheme are summarized.