This book describes the properties and device applications of hydrogenated amorphous silicon. It covers the growth, the atomic and electronic structure, the properties of dopants and defects, the optical and electronic properties which result from the disordered structure and finally the applications of this technologically very important material. There is also an important chapter on contacts, interfaces and multilayers. The main emphasis of the book is on the new physical phenomena which result from the disorder of the atomic structure. The book will be of major importance to those who are researching or studying the properties and applications of a-Si:H. It will have a wider interest for anyone working in semiconductor physics and electronic engineering in general.