A wide range of geophysical and geological data indicate that extensive serpentinization in the forearc mantle is both expected and observed. Large volumes of aqueous fluids must be released upwards by dehydration reactions in subducting oceanic crust and sediments. Subduction of oceanic lithosphere cools the overlying forearc such that low temperature hydrous serpentine minerals are stable in the forearc mantle. Over several tens of millions of years estimated fluid fluxes from the subducting plate are sufficient to serpentinize the entire forearc mantle wedge. However, fluid infiltration is probably fracture controlled such that mantle serpentinization is heterogeneous. Geological evidence for hydration of the forearc mantle includes serpentine mud volcanoes in the Mariana forearc and serpentinites present in exposed paleo-forearcs. The serpentinization process dramatically reduces the seismic velocity and density of the mantle while increasing Poisson's ratio. Serpentinization may generate seismic reflectivity, an increase in magnetization, an increase in electrical conductivity, and a reduction in mechanical strength. Geophysical evidence for serpentinized forearc mantle has been reported for a number of subduction zones including Alaska, Aleutians, central Andes, Cascadia, Izu-Bonin-Mariana, and central Japan. Serpentinization may explain why the forearc mantle is commonly aseismic and in cool subduction zones may control the downdip limit of great subduction thrust earthquakes. Flow in the mantle wedge, induced by the subducting plate, may be modified by the low density, weak serpentinized forearc mantle. Large volumes of H 2O may be released from serpentinized forearc mantle by heating during ridge subduction or continent collision.