Grain boundaries in crystalline solids exert a pronounced influence on almost all the properties of polycrystalline bodies, as well as affecting the kinetics of most solid-state processes that occur therein. They have therefore been the subject of numerous theoretical and experimental researches in both metallic and non-metallic solids. Previous reviews of grain boundaries 1-12 have emphasized chiefly structure and some general effects of grain boundaries on properties. Here, attention will be confined to certain aspects of the chemistry of grain boundaries and grain-boundary regions. Although in many instances analogous phenomen may occur at other structural anomalies, such as coherent twin boundaries, mosaic walls, slip bands, individual dislocations, interphase boundaries, antiphase boundaries, or free surfaces, little explicit treatment of these topics will be included.