Pegmatites are texturally complex igneous rocks marked by some combination of coarse but variable crystal size, mineralogical zonation, prominent anisotropy of crystal orientations from the margins inward, and skeletal, radial, and graphic intergrowth habits of crystals. The vast majority of pegmatites are granitic in composition, and this article pertains to these rocks. Pegmatites occur as segregations near the roofward contact of their source pluton, as dike swarms emanating from their plutons into the surrounding igneous and metamorphic rocks, and as planar to lenticular intrusive bodies whose sources are not exposed. Granitic pegmatites are important economic sources of industrial minerals (feldspars, quartz, spodumene, petalite) for glass, ceramic, and electronic applications, of a wide variety of lithiophile rare elements (Li, Cs, Be, Nb, Ta, Sn, etc.) that are incompatible in the predominant rock-forming minerals of granites, and of colored gemstones and valuable mineral specimens (of beryl, tourmaline, topaz, etc.). All of the salient features of pegmatites – their mineral habits, distinctive rock fabrics, and spatial zonation of mineral assemblages, including monominerallic bodies – arise from appreciable liquidus undercooling (by ∼200° ± 50 °C) of viscous granitic liquids prior to the onset of crystallization. The ore-forming processes within granitic pegmatites are entirely igneous as a result of extended fractional crystallization of large granitic plutons, and in response to crystallization at a highly supersaturated state of the melt in pegmatite-forming bodies. Constitutional zone refining, wherein a flux-enriched boundary layer of liquid develops adjacent to the crystallization front and eventually becomes the last liquid in the pegmatite body, appears to be the most likely mechanism to account for increasing crystal dimensions (and consequently decreasing numbers of crystals) and late-stage bodies of rare-metal ores that are the hallmarks of the class of rare-element pegmatites. The most preva...