This is the first book to give a complete overview of the properties of deep-level, localized defects in semiconductors. Such comparatively long-lived (or metastable) defects exhibit complex interactions with the surrounding material, and can significantly affect the performance and stability of certain semiconductor devices. After an introductory discussion of metastable defects, the properties of DX and EL2 centres in IIIV compounds are presented. Additional crystalline materials are also dealt with, before a detailed description is given of the properties and kinetics of photo-induced defects in amorphous semiconductors. The book closes with an examination of the effects of photo-induced defects in a range of practical applications. Throughout, unifying concepts and models are stressed, and the book will be of great use to graduate students and researchers interested in the physics and materials science of semiconductors.