The subject of this book is the investigation of lattice imperfections in semiconductors by means of positron annihilation. A comprehensive review is given of the different positron techniques, whose application to various kinds of defects, e.g. vacancies, impurity-vacancy complexes and dislocations, is described. The sensitivity range of positron annihilation with respect to the detection of these defects is compared to that of other defect-sensitive methods. The most prominent results obtained with positrons in practically all important semiconductors are reviewed. A special chapter of the book deals with positron annihilation as a promising tool for many technological purposes. The theoretical background necessary to understand the experimental results is explained in detail.