Interdiffusion between coexisting minerals affects all rocks and causes resetting and discordance of stable isotope geothermometers that is commonly observed in slowly cooled igneous and metamorphic rocks. The Fast Grain Boundary (FGB) model describes the stable isotope fractionations and intracrystalline zonation which result from closed system interdiffusion (Eiler et al., 1991, 1992). This model assumes that grain boundary diffusion is much faster than volume diffusion, and it accounts for exchange among all minerals in a rock. Previous models of closure temperature violate mass balance restrictions and will be inaccurate in most rocks. Modeling results are described for amphibolites and hornblende granites and gneisses; biotite granites, schists, and gneisses; pelitic and semi-pelitic rocks; garnet peridotites; anorthosites, gabbros, pyroxenites, and related rocks; and calc-silicate rocks. Examples of mineral pairs and specific rock types that allow accurate stable isotope thermometry include plagioclase-pyroxene in pyroxene bearing anorthosites and garnet-quartz in garnetiferous quartzites. In contrast, the same mineral pairs in related rocks such as pyroxenites and pelitic schists will exhibit reset apparent temperatures. Closed-system processes are capable of producing a variety of patterns of stable isotope resetting, discordance, mineral zonation, and fractionation reversals. Examples include large reversals of quartz-feldspar fractionations in micaceous rocks, and oscillatory zonation in feldspar from some quartz-rich rocks. These results permit reinterpretation of many studies of stable isotope thermometry, speedometry, and retrograde alteration history. FGB modeling of mineral zonation provides an important new guide to applying recently developed micro-analytical tools to slowly cooled rocks. Application of the FGB model to quartzo-feldspathic gneisses from the Adirondack Mountains, New York, demonstrates the usefulness of diffusion modeling in discriminating closed-system, diffusion controlled retrogression from open-system retrogression, and illustrates the possible importance of incorporating the effect of water activity on mineral diffusivity.