Research on glassy semiconductors continues to expand every year. This is evidenced by the ever-increasing number of articles devoted to glassy semiconductors and published in a great variety of periodicals. The time has come to systematize and generalize the abundant published experimental material. The first review of the experimental data on glass formation and the physicochemical and physical properties of chalcogenide glassy semi conductors was published by B. T. Kolorniets [1]. Glass formation in chalcogenide systems is the subject of a section in a monograph by Rawson [2]. In 1972 the Leningrad University published the author's books [3] dealing with the regularities of glass formation in cha1cogenide systems and containing a systematized exposition of some physicochemical properties of glassy cha1cogenide semiconductors. The monograph presented mainly results of research performed by the Semiconductor Chemistry Laboratory staff of the Leningrad University. These investigations were started at the initiative and under the dircction of Professor R. L. Myuller and wcre continued under the author's direction. The present monograph is a revised and substantially supplementcd version of the aforementioned publication. However, the extensive experimental material in the literature is far from completely presented. It contains mainly data on the research performed by the staff of the laboratory headed by the author. However, data obtained by other Soviet and foreign workers are represented to a greater degree in this book than in the preceding edition.