Abstract Giant porphyry molybdenum deposits are best exemplified by the Climax and Henderson deposits in Colorado. The high grades of these deposits are probably inherited from magmatic molybdenum concentrations of about 4 to 5 ppm, which are high for metaluminous rhyolitic magmas that average about 2 ppm molybdenum. High magmatic molybdenum concentrations in metaluminous rocks appear to be related to high magmatic oxygen fugacities (2 or 3 log units above QFM oxygen buffer) and are correlated with high niobium concentrations. High oxygen fugacities are likely inherited from calc-alkaline or lamprophyric predecessors. High niobium and molybdenum are related to extreme fractionation of rhyolitic magmas. Much higher concentrations of molybdenum (> 1,000 ppm) in the ore fluid (and the cupola magma) are probably achieved by crystallization in the deeper portions of a magma chamber accompanied by convection of the evolved liquid to the cupola and volatile fluxing. Exploration criteria for a giant, high-grade deposit include: 1) a tectonic setting that indicates a changeover from compressional to extensional tectonics, 2) thick continental crust at the time of deposit formation may encourage extreme differentiation and crustal contamination, 3) an isotopically zoned magma chamber indicative of a long-lived heat source, 4) a large, sub-volcanic, central-vent ash flow/dome system that erupted less than 100 km3 of rhyolite, and 5) high niobium concentrations (> 75 ppm) in a subalkaline, magnetite-bearing rhyolite.