Book excerpt: This book focuses on the deliberate introduction and manipulation of defects and impurities in order to engineer desired properties in semiconductor materials and devices. In view of current exciting developments in wide-bandgap semiconductors like GaN for blue light emission, as well as high-speed and high-temperature electronics, dopant and defect issues relevant to these materials are addressed. Also featured are semiconductor nanocavities and nano-structures, with emphasis on the formation and impact of vacancy-type defects. Defect reaction problems pertaining to impurity gettering, precipitation and hydrogen passivation are specific examples of defect engineering that improve the electronic quality of the material. A number of papers also deal with characterization techniques needed to study and to identify defects in materials and device structures. Finally, papers also address issues such as interface control and passivation, application of ion implantation, plasma treatment and rapid thermal processing for creating/activating/suppressing trap levels, and device applications.