The lectures and seminars recorded in Amorphous Insulators and Semiconductors fill the gap between graduate courses and research seminars. The book is organized into four sections, starting with general considerations of glass forming ability and techniques for the preparation of different kinds of glasses. There has been activity on the nature of the glass transition in recent years, but it remains a difficult and poorly understood subject. The second section discusses the various techniques used to investigate the structure of the glasses, including extensive notes on computer simulation techniques. The third section goes into vibrational states, with special emphasis on the interaction between theory and experiment. In the final section the electronic structure of the glasses is reviewed, together with its relationship with their electrical properties, which involves an understanding of defects. Also included in this section is a review of current thinking on tunnelling states in glasses.