The pegmatite province of the Southeastern Desert (SED) is part of a district that extends from eastern Egypt to the Red Sea (and covers an area of approximatly 120 km2). Rare-metal pegmatites within the SED are divided into three major types: (1) unzoned, Sn-mineralized bodies; (2) zoned Li-, Nb-, Ta-, and Be-bearing bodies; and (3) pegmatites containing colored, gem-quality tourmaline. Rb-Sr isotope data reflect a crustal origin for the rare-metal pegmatites and indicate that the original SED magma was generated during the regional metamorphic peak predating intrusion of posttectonic leucogranites. These bodies developed an early border zone consisting of coarse to very coarse-muscovite + quartz + alkali feldspar, followed by an intermediate zone of dominantly quartz + feldspar + muscovite rock. Garnet, tourmaline, beryl, galena, pyrite, amblygonite, apatite, and monazite are rare accessories in both zoned and unzoned pegmatites. Cassiterite occurs in replacement zones and along fractures in albite + quartz + muscovite-rich portions. The highest concentrations of coarse-grained cassiterite occur in irregular greisenized zones that consist dominantly of micaceous aggregates of green Li-rich muscovite, quartz, and albite. Different metasomatic post-solidification alterations include sodic and postassic metasomatism, greisenization and tourmalinization. The pegmatite-generating granites are metaluminous, showing a differentiation trend from coarsegrained, unfractionated, intermediate plagioclase-rich granite toward a highly fractionated, fine- to medium-grained, locally albite-rich rock. Economically important ore minerals were introduced by rare metal-bearing fluids—either primarily or during the breakdown of the primary mineral assemblages. These include niobiumtantalum oxides, Sn-oxides (cassiterite), Li-silicates (petalite, spodumene, euctyptite, and pollucite), Li-phosphates (amblygonite, montebrasite, and lithopilite), and minor REE minerals (Hfzircon, monazite, xenotime, thorian, loparite, and yttrio-fluorite). Pollucite is typically associated with spodumene, petalite, amblygonite, quartz, and feldspar. Primary pollucite has Si/Al ratios of 2.53 to 2.65 and CRK (Cs + Rb + K/Cs + Rb + K + Na + Ca + Li) of 79.5 to 82.2. Thorian loparite is essentially a member of the loparite (NaLREETi2O6)-lueshite (NaNbO3)-ThTi2O6-ThNb4O12 quaternary system with low or negligible contents of other end-members. The mineral compositionally evolves from niobian loparite to niobian thorian and thorian; loparite gave rise to ceriobetafite and belyankinite with high ThO2 content. Thorian loparite is metamict or partly metamict, and upon heating regains a structure close to that of synthetic loparite, NaLaTi2O6.