This book celebrates 20 years of MRS symposia on the topic of amorphous silicon. Contributors showed that the simplified theories developed to explain the limited experimental information available in the early eighties have spurred more sophisticated experimentation - either refining the early understanding or making it irrelevant. The differences of opinion that continue to exist and emerge are probably the hallmark of the amazing vitality of this field. Applications range from 'mature' thin-film transistors, solar cells and image sensors, to the 'emerging' possibility of erbium-doped nanocrystalline silicon-based materials for lasers and amorphous silicon quantum dots for luminescent devices. The book discusses material characterization, growth processes and devices. Each chapter is further subdivided into sections that group papers around common themes. Topics include: nanomaterials; electronic structure; metastable effects; understanding of growth processes; laser-induced crystallization; metal-induced crystallization; other growth techniques; newer devices; solar cells and thin-film transistors.