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.