Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 31.This volume grew from a Chapman Conference held at Fallen Leaf Lake, California, September 5–9, 1982, on the topic “Point Defects in Minerals.” The aim of this conference was to bring together a variety of experts within the geosciences and those disciplines (primarily solid-state physics and chemistry and materials science) that have traditionally studied the role of point defects in solids. Defects exist in all crystals at temperatures above absolute zero and arise from the tendency of crystal structures to disorder with increasing temperature or from chemical substitution. In the absence of outside forces, the most common form of defect is one in which an atom moves from its position in the perfectly symmetric crystal to some other position not normally occupied by this atom, or perhaps any atom. Such defects are termed point defects and they are ubiquitous, although at temperatures below melting their concentration rarely exceeds several percent of the total number of atoms. Nevertheless, point defects may either control or actively participate in many physical and chemical processes in minerals, processes which in turn are important to the formation and evolution of the earth. The need to solve problems posed by recent advances in the geosciences (e.g., the development of the concept of plate tectonics) requires the extension of present concepts in point defect theory, worked out on relatively simple systems, to materials far more complex, such as the silicate minerals found in the earth.