This book presents the current level of understanding of the structural, electronic and optical properties of amorphous semiconductors. As amorphous materials depart significantly from the crystalline counterparts, some of the basic problems associated with the validity of the effective mass approximation, whether K is a good quantum number, and concepts of phonons and excitons will be addressed in detail. A significant part of the book is devoted to present recent progress made in the understanding of light-induced degradations in amorphous semiconductors, which is regarded as the most limiting problem in device applications. The monograph presents a comprehensive review of both experimental and theoretical studies on amorphous semiconductors.