In the past 12-15 years an essentially new trend in electrochemistry has sprung up around the problem of solar energy conversion. Strictly speaking, this is not a purely electrochemical but an interdisciplinary field involving the fields of cataly­ sis, corrosion, chemistry of disperse systems, and others. Nevertheless, electro­ chemistry, to be more exact, photoelectrochemistry of semiconductors, provides a theoretical basis for new methods of converting light energy into electrical or chemical energy, which, we hope, shall find practical application in the not so dis­ tant future. In the past years, this field has been discussed amply and at length in special monographs (e. g. , in Ref. [l]). Therefore, in this book the photoelectro­ chemistry of semiconductors is presented in a concise form (exceptions are only specific problems which have been elucidated incorrectly or have not been covered completely in the literature). In this compact monograph we have aban­ doned the principle of "self-seclusion": for a more deep insight into the funda­ mentals of electrochemistry, photoelectrochemistry, and physics of semiconduc­ tors the reader shall have to refer to the below-cited manuals, while information on the physicochemical properties of particular semiconductor electrodes can be taken, e. g. , from Refs. [2, 3].