The metamorphic evolution and dehydration of subducting oceanic crust may be predicted by combining calculated pressure-temperature (P-T) paths with a model of metabasalt phase equilibria. In steady-state subduction zones with high rates of shear heating, the upper parts of the subducting oceanic crust progress through the greenschist → amphibolite → granulite → eclogite facies, whereas lower parts of the subducting oceanic crust progress through the blueschist → eclogite facies. In steady-state subduction zones with moderate rates of shear heating, most of the subducting oceanic crust passes through the blueschist → eclogite transition. In steady-state subduction zones with low rates of shear heating, the entire subducting oceanic crust lies within the blueschist facies to depths greater than 70 km. For oceanic crust containing 1-2 wt% H2O, dehydration will not begin until the onset of eclogite- or amphibolite-facies metamorphism, depending on the P-T path. For many subduction zones, the most important dehydration reactions in the subducting oceanic crust occur at the blueschist → eclogite facies transition associated with the breakdown of lawsonite (or clinozoisite), glaucophane, and chlorite. Large amounts of H2O released by blueschist → eclogite dehydration reactions could trigger partial melting in the overlying mantle wedge and may play a crucial role in the generation of arc magmas.