Arc volcanism is intimately linked to mineral dehydration reactions in the subducting oceanic mantle, crust, and sediments. The location of slab dehydration reactions depends strongly on the temperature and pressure conditions at the top of the subducting plate and hence on the detailed thermal structure of subduction zones. A particularly important physical property of subduction zone thermal models is the viscosity of the mantle wedge. The introduction of an olivine rheology, with appropriate stress and temperature dependence, focuses flow into the tip of the mantle wedge. This leads to a temperature increase of a few hundred degrees in the wedge and the top of the slab as compared to the isoviscous case. Sensitivity tests show that this conclusion is robust under a variety of subduction zone parameters. The new high-resolution finite element models are used to reevaluate the thermal structure of the Honshu and Cascadia subduction zones using the more realistic olivine rheology. For Honshu, the model predicts slab--mantle interface temperatures of ~800¿C beneath the volcanic front. Deeper parts of the subducting oceanic crust and upper mantle remain relative cool (e.g., the subducted Moho beneath the volcanic front is ~400¿C) because of the rapid subduction of old Pacific lithosphere. High interface temperatures are consistent with Th and Be data, indicating sediment melting, whereas cooler crustal temperatures are consistent with boron evidence, indicating lower temperature dehydration. The strong temperature gradient at the top of the subducting plate may thus reconcile conflicting estimates of slab temperature in subduction zones that are characterized by rapid subduction of mature oceanic lithosphere. Low temperatures persist to great depth in the shallow upper mantle of the subducting plate, which allows for water transport in the form of hydrous minerals to the deep mantle. The thermal structure of the Cascadia subduction zone is markedly different because of the age of the incoming lithosphere and low subduction speed. In this case, shallow dehydration and melting of the subducting crust and lithosphere is predicted.