This review discusses the structure of an electrical double layer formed at the semiconductor-electrolyte interface, as well as the energy states and imperfections in the electrolyte and on the semiconductor surface. A considerable part of the review is devoted to the results of investigations of photoelectric effects, luminescence, reflection, and electroreflection at this interface. The quantum size effects which occur in semiconductor electrodes and in colloids and the injection of hot electrons into an electrolyte are discussed in later sections of the review. A classification of photoelectrochemical cells is given and current data are provided on devices for conversion of solar or laser radiation energy into electrical and chemical energy.