Recognition of the importance of fluid flow in the process of metamorphism was an outgrowth of efforts by petrologists over the last 50--60 years to understand the mineralogical evolution of metamorphic rocks. Mineralogical, chemical, and isotopic data are now routinely used to identify where fluid has flowed in metamorphic terranes, to measure the amount of fluid, to constrain the direction of flow relative to temperature and pressure gradients and lithologic contacts, and to determine the age of flow. Fluid may flow through a static, interconnected network of microscopic tubes at grain corners only under special combinations of mineralogy, fluid composition, pressure, and temperature. Fluid flow typically is restricted to hydraulic fractures whose formation and maintenance require dynamic processes such as increasing temperature, active devolatilization reactions, and/or deformation. Hydraulic fracture flow is heterogeneous in both space and time. Metamorphic fluid transport may be driven by density differences between rock and fluid, by density variations in fluid generated from temperature gradients, by deformation, and by surface tension. Metamorphic fluid flow plays a significant role in heat and mass transfer in Earth's crust and in the mechanisms and rates of deformation. ¿ American Geophysical Union 1994