Metamorphic devolatilization generates fluid and grain-scale porosity. Evidence for high fluid pressure indicates that devolatilization occurs under poorly drained conditions. Under such conditions, fluid expulsion is limited by the capacity of the reacted rocks to resist compaction or by the rate at which deformation modifies the permeability of the overlying rocks. In the former case, the compaction timescale must be greater than the metamorphic timescale, and flow patterns are dictated by details of rock permeability. The alternative is that compaction processes are fast relative to metamorphism. In this case, flow is compaction driven and accomplished by waves of fluid-filled porosity.