This paper reviews, on a world-wide basis, the production from hydro-electric installations and forecasts which have been made of potential water-power resources. Various types of development are described. It considers how hydro-electric resources can best be used in an interconnected electricity supply system with thermal and nuclear generation, including pumping to assist in the economic use of other fuels. Hydro-electric development in the United Kingdom is briefly described. The availability of tidal energy is reviewed, and reference made to the Ranee Tidal Power Station in France.