The generation of pegmatite dikes during the cooling and crystallization of granitic plutons has been calculated using new models for the prediction of granitic melt viscosities and the propagation of dikes. These new models suggest that early in the cooling history of a modeled 10X10X10 km pluton, dikes cannot propagate, or will be short (on the order of 1 km), because the surrounding country-rocks have not yet been significantly heated. However, dikes formed tens to hundreds of thousands of years after intrusion can propagate up to approximately 10 km. Because the far-propagating dikes form late in the magmatic history of the pluton, they will be composed of chemically more evolved magmas than the bulk of the pluton and will crystallize as pegmatites. The model predicts that pegmatites should only rarely be found more than ca. 10 km from their host pluton, that more-evolved pegmatites should be found at greater distances from their host pluton than less-evolved ones, and that pegmatites should not be associated with small plutons. All of these model results are consistent with field observations, and support the petrogenetic relationship between granitic plutons and the evolved pegmatites surrounding them.