This book surveys the theory of defects in solids, concentrating on the electronic structure of point defects in insulators and semiconductors. The relations between different approaches are described, and the predictions of the theory compared critically with experiment. The physical assumptions and approximations are emphasized. Theory of Defects in Solids begins with the perfect solid, then reviews the main methods of calculating defect energy levels and wave functions. The calculation of observable defect properties is discussed, and finally, the theory is applied to a range of defects that are very different in nature. This book is intended for research workers and graduate students interested in solid-state physics.