A variety of sediment and crustal deformation, associated with fluid activity, has been observed in the present forearc slopes, trenches and ancient onland outcrops in the Japanese island arcs. The Nankai forearc represents a typical clastic-dominated accretionary prism where the expulsion of pore fluids from sediments seems to have occurred intermittently, through both channelized and diffusive mechanisms, some of which appears to be pulsed. Mud diapirs occur within the trench, near the toe and upper slope of the accretionary prism. The widespread development of a gas-hydrate phase boundary (bottom simulating reflector (BSR)) may indicate pervasive fluid advection throughout the prism. The Japan Trench forearc is characterized by tectono-gravity slope instability and collapse, resulting in zones of fluid escape along normal faults. No BSR is observed in the Japan forearc despite the presence of organic-rich sediments. The Izu-Bonin forearc, apparently unlike the Japan and Nankai Trench forearcs, contains recently discovered serpentinite diapirs in which there are blocks of basic-ultrabasic rocks, together with blueschist. Fossil Calyptogena beds in the Pliocene basins of the Izu Collision Zone provide good examples of fluid activity associated with venting in an ancient accretionary prism now exposed onland. Contrasting tectonic processes in the three forearcs may be explained by differences in the plumbing systems and hydrogeologic characteristics of the forearc basements: dewatering sediments in the Nankai Trough prism; impermeable and consolidated sediments in the Japan Trench prism, and the very permeable pillow-lava-volcaniclastic complex of the Izu-Bonin Trench forearc.