The basic requirement of the control system is to ensure the supply of electric power at constant voltage and frequency exactly in accordance with the instantaneous demand. Its design is dominated by (a) the characteristics of the consumers' demand; (b) the provision of a reliable supply, even under fault conditions, at minimum cost; (c) the technical and operational characteristics of a mixture of fossil-fired, nuclear and hydro generators; (d) an assessment of the benefits to be gained and the cost of implementation. Large modern generating units are complex and their characteristics and those of the transmission system are such as to increase the difficulties of operation and control beyond the unaided capabilities of human operators. Automatic aids, many of them computer-based, are being used increasingly. The paper reviews the present position and indicates how advances in control technology could lead to on-line control of the system at acceptable cost.