Crystal defects can no longer be thought of as a scientific curiosity, but must be considered an important aspect of solid-state science. This is largely because many of the more interesting properties of crystalline solids are disproportionately dominated by effects due to a tiny concentration of imperfections in an otherwise perfect lattice. The physics of such lattice defects is not only of significance in a great variety of applications, but is also interesting in its own right. Thus, an extensive science of point defects and dislocations has been constructed during the past two and a half decades. Stimulated by the technological and scientific interest in plasticity, there have appeared in recent years rather a large number of books dealing with dislocations; in the case of point defects, however, only very few broad and extensive treatments have been published. Thus, there are few compre­ hensive, tutorial sources for the scientist or engineer whose research ac­ tivities are affected by point defect phenomena, or who might wish to enter the field. It is partially to fill this need that the present treatise aims.