Postcruise scientific investigations based on the results of Leg 170 have made significant inroads into our understanding of flow systems in the largely nonaccretionary subduction setting of the northern Costa Rica subduction zone. With constraints from heat flow, geochemistry, seismic imaging, and the physical properties and fabrics of core materials, we are gaining an understanding of the paths and mechanisms of flow through the decollement and margin wedge, the underthrust sediments, and the uppermost basement of the lower plate. Significant discoveries include estimates of the depth of origin of decollement fluids, rates of flow through the underthrust sediment and the uppermost oceanic crust, the hydraulic separation of the sediment from the upper-most crust, and constraints on the mechanisms driving flow in the basement. These discoveries have opened new avenues for future research.