Rapid dewatering, largely through loss of porosity, is one of the most striking changes affecting deformed sediments in accretionary prisms. Analysis of porosity from sample and seismic data confirms that porosity in accretionary prisms is significantly less than that in basinal strata of similar lithology. More specifically, the porosity depth gradient increases arcward across the prism, until porosities less than 10% occur within a few kilometers of the surface. In deeper parts of the prism, dewatering and volume reduction occur not only during loss of porosity but also during dehydration and metamorphic reactionss. Calculations involving volume change lead us to define a function, &Ggr;, which reflects total volume reduction. The distribution of the relative intensity of dewatering within the prism can be calculated from the porosity contours and trajectories of sediment elements. An initial attempt indicates that in the toe region the rate of dewatering is accelerated at depth relative to the basinal section and may be greatest near the base of the prism. Toward the rear of the prism the maximum rate of dewatering occurs at progressively shallower depths. Several lines of evidence suggest that, in addition to gradual diffuse dewatering, localized zones of strong dewatering are associated with zones of intense shear. This model of shear dewatering, based on critical state soil mechanics, suggests that sediments adjacent to the basal decollement should be strongly dewatered.