Ocean Drilling Program (ODP) Leg 205 was designed to investigate subduction fluxes and fluid flow across the Costa Rica convergent margin. During Leg 205, modified CORK-IIs (new-generation Circulation Obviation Retrofit Kits) were installed within the incoming plate and the decollement zone to monitor pressure and temperature of formation fluids through time and to collect a time series of fluid samples for chemical analysis. One subseafloor observatory is monitoring within a highly fractured region of the incoming igneous section. Vigorous, slightly subhydrostatic flow within high-permeability horizons is observed; fluid compositions appear to be a mixture of bottom seawater and components distinct from seawater. The observatory within the decollement zone reveals pressures that are just slightly overpressured; excursions in the pressure and temperature record correlate temporally with excursions in fluid composition and flow rate variation, all occurring during periods of aseismic strain, recorded geodetically. Igneous and sedimentary core materials collected during Leg 205 have been studied extensively in combination with materials recovered during ODP Leg 170. This synthesis and papers in this volume report on the igneous and alteration history of the incoming oceanic section and its implications for off-axis magmatism and fluid flow, the composition of incoming and underthrust sediments with implications for fluid and element loss from the shallow subduction zone, the structure and permeability characteristics of the forearc sediment wedge, and the consequences of sediment dynamics and fluid flow to the seismogenic zone and subduction factory.