The construction of hydroelectric power plants is always done using geological information and frequently the whole project is conditioned by the geological nature and characteristics of the site. The importance of geology in the planning, construction and operation of hydroelectric power plants will be presented and discussed. Without trying to be exhaustive, some aspects related to the watertightness of the reservoir, the stability of the reservoir slopes, the watertightness and deformability of the dam foundations, site conditions of the appurtenant structures and availability of construction materials, will be presented.