This book is devoted to the specific physical and chemical properties of centers in semiconductors with shallow energy levels and electronic distributions of an extended size. Reports are included on the most advanced experimental and theoretical methods for identifying and further characterizing these materials. Attention is given to such topics as shallow-level centers in host semiconductors of lower dimensionality, centers in wide-bandgap semiconductors, shallow excited states of centers with deep ground states, passivation of centers, and other aspects of impurity control during crystal growth and processing with its relevance to applications.