Material recycling in subduction zones, including generation and migration of aqueousfluids and melts, is key to understanding the origin of volcanism in subduction zones. It isalso important for understanding the global mass budget, such as formation and growth ofthe continents. Recent knowledge concerning the phase relationships of hydrous peridotiticand basaltic systems allows us to model fluid generation and migration in subduction zones.Here we present (1) numerical models for the transportation of H₂O and melting beneath theJapanese islands, in which generation and migration of aqueous fluid, its interaction with theconvecting solid, and melting are considered, based on the phase relationships, (2) predictionsof the corresponding seismic structures based on the calculated distribution of the fluids (aqueous fluids and melts), (3) 3-D seismic tomographic images beneath the islands, (4) analyses on distribution of the volcanoes and the volcanic chains, and (5) comparisonsbetween the model predictions and the observations.<BR>The model calculation suggests that in northeast Japan, nearly all the H2O expelled fromthe subducted Pacific plate is hosted by serpentine and chlorite just above the plate, and isbrought down to 150-200km. Breakdown of serpentine and chlorite at these depths results inthe formation of a fluid column through which H₂O is transported upwards, and results inthe initiation of melting in the mantle wedge beneath the backarc. The seismic tomographicstudies suggest the existence of such a melting region beneath the backarc. In central Japan, the subducted Philippine Sea plate overlaps the subducted Pacific plate. This geometry causesslow thermal recovery of the subducted Pacific plate, resulting in dehydration reactions atlevels (200-300km) deeper than in northeast Japan, and bending of the volcanic chaintowards the backarc side. In contrast, in southwest Japan, where a relatively hot part of thePhilippine Sea plate (Shikoku Basin) subducts, the dehydration reactions are predicted tooccur at relatively shallow levels (100km depth). The seismic tomographic image supportswell the predicted distribution of the fluids beneath the volcanic front to the forearc region.These comparisons between the model predictions and the observations suggest that th ethermal structure (or age) of the subducting plate strongly controls the distribution of the aqueous fluids and melts in subduction zones through the position of the dehydration reactions.