Point defects exist in all crystals at temperatures above absolute zero. They arise either to prevent charge imbalances d u e to the presence of impurities or because thermal energy has caused atoms to be displaced from their normal positions in th e structure. T h e atoms are free to migrate within the crystal, and this migration provides an energy-efficient mechanism for the movement and mixing of atoms. This movement results in mass transport, or diffusion, and in the attainment of thermal equilibrium. If the defect can be defined in terms of a site in a crystal lattice, it is called a point defect. More complex defects, such as dislocations, grain boundaries, or inclusions, can be considered as extensions of point defects in one or more dimensions. Point defects can be treated as chemical entities, and their behavior can be quantitatively predicted by various theories. Their role in minerals can thus be predicted, although the minerals of most interest, the silicates, are chemically and structurally complex enough to render the work tedious. Nevertheless, many techniques developed in other fields are now being applied to understanding the behavior of point defects in minerals that are important to the evolution of the earth and the terrestrial planets.