There is ''too much'' water in a variety of crustal environments, including sedimentary basins and regional metamorphic belts. Regional and contact metamorphic rocks have time-integrated fluxes of 106 cm3 water/cm2 for pervasive low and up to 109 cm3 water/cm2 in fracture flow, amounts far greater than can be supplied by dehydration of immediately subajacent rocks. The types of evidence supporting high water fluxes are so numerous and varied, including petrologic, mineralogic, textural, microstructural, and stable isotopic data, that it is unlikely that all of them could have been interpreted erroneously. Thus the question arises as to what mechanisms of permeability enhancement might operate to facilitate large fluid fluxes. Field evidence suggests the operation of reaction enhancement of permeability, grain-scale dilatancy, and hydraulic fracturing during metamorphism. All these mechanisms are inherently episodic and transient. The transport properties of dynamic permeability enhancement are poorly known in relation to the thermal and deformational cycles of metamorphism. Future research should be directed toward experimental and theoretical simulation of permeability enhancement with close collaboration between isotope geochemists, structural geologists, and petrologists to test models of permeability enhancement in the field. ¿ American Geophysical Union 1994