In introductory solid-state physics texts we are introduced to the concept of a perfect crystalline solid with every atom in its proper place. This is a convenient first step in developing the concept of electronic band struc ture, and from it deducing the general electronic and optical properties of crystalline solids. However, for the student who does not proceed further, such an idealization can be grossly misleading. A perfect crystal does not exist. There are always defects. It was recognized very early in the study of solids that these defects often have a profound effect on the real...