Structural crystallography has been an active field of endeavor ever since the pioneering studies of M. Laue and the Braggs (1912–1913). Since that time the basic methods for solving crystal structures have been applied to many disciplines, including biology, chemistry, mineralogy, and physics. Until about ten years ago, structural crystallography was applied to mineralogy largely by solving new crystal structures, and the results helped greatly in the understanding of the chemistry and physical properties of minerals. The boundary conditions of such studies were usually clearly defined by the solution of the crystal structure, i.e., locating the atoms in the crystal unit cell. In fact, the boundary conditions were so clearly defined that in many geology departments the crystallography laboratory stood as an isolated unit with little interaction with the rest of the department. The crystal structures of most of the important rock-forming minerals are now solved, and crystallography is taking on a new role in geology. Important technological advances involving automated X-ray diffractometery, high-speed digital computers, and new and better spectral techniques enable the crystallographer to study many crystals in a reasonable amount of time and obtain important information on the dependence of the structure on chemical substitutions, cation distributions over crystallographically distinct sites (intracrystalline equilibria), details of bonding, and high-temperature phase transitions. Many of these results are of great interest and importance to the petrologist. The result has been important interaction between crystallography and petrology and hence ‘petrologic crystal chemistry.’ What should or should not be included in the U.S. contribution to this field is somewhat arbitrary, but perhaps the first scientific session on this subject at an AGU meeting in 1967 entitled ‘Properties of Crystalline Solutions’ can set the tone. Over the last four years other scientific sessions at national meetings and smaller conferences have emphasized ‘petrologic crystal chemistry,’ and not only crystallographers, but experimental, field, and theoretical petrologists have attended and contributed. The following discussion and bibliography is an attempt to point out some of the important areas of activity. In keeping with the tradition of the volcanology, geochemistry, and petrology section of AGU the emphasis will be on igneous and metamorphic minerals and, more specifically, on the rock-forming silicates. Selected abstracts are included in the bibliography, but only published papers are considered in the discussion.