Unique among the terrestrial planets, the Earth has a substantial hydrosphere, which has had an extraordinary influence on the way the planet has evolved. Whereas volcanic activity is the main route by which fluids escape from depth to reach the atmosphere/hydrosphere, the dominant route by which fluids may enter the deeper levels of the Earth is via subduction zones. How deep can these fluids penetrate? More importantly, what influence do fluids have in controlling geological processes at depth? This volume contains the proceedings of the Discussion Meeting, held at the Royal Society in November 1990, which tried to assess the role of fluids in the subduction environment, first at shallow levels where material is entering the subjuction system, but then following processes down to progressively deeper levels in the Earth"s mantle, to the zone of melting and beyond. These processes include the formation of accretionary sedimentary prisms, which represent important lateral additions to the continental crust at island arcs and continental margins, and whether subduction zones represent the important interface across which some of the major chemical reservoirs in the Earth are formed. This collection of papers serves as both a review or synthesis of what recent research in geophysics, oceanography, sedimentology, tectonics, geochemistry, mineralogy, petrology and thermal modelling has achieved in recent years and points to the direction of future investigations.