This book covers electronic and structural properties of light-induced defects, light-induced defect creation processes, and related phenomena in crystalline, amorphous, and microcrystalline semiconductors. It provides a theoretical treatment of recombination-enhanced defect reaction in crystalline semiconductors, particularly GaAs and related materials. It also discusses experimental evidence for this phenomenon. Light-induced defect creation in hydrogenated amorphous silicon (a-Si: H) is described in more detail, including its mechanism and experimental results. The subjects treated by the book are important issues from the viewpoints of physics and applications.