In the upper crust, where hydraulic gradients are typically <1 MPa km-1, advective heat transport is often effective for permeabilities k ≥ 10-16 m² and advective mass (solute) transport for k ≥ 10-20 m². Regional-scale analyses of coupled groundwater flow and heat transport in the upper crust typically infer permeabilities in the range of 10-17 to 10-14 m², so that heat advection is sometimes significant and solute advection should nearly always be significant. Analyses of metamorphic systems suggest that a geochemically significant level of permeability can exist to the base of the crust. In active metamorphic systems in the mid to lower crust, where vertical hydraulic gradients are likely >10 MPa km-1, the mean permeabilities required to accommodate the estimated metamorphic fluid fluxes decrease from ∼10-16 m² to ∼10-18 m² between 5- and 12-km depth. Below ∼12 km, which broadly corresponds to the brittle-plastic transition, mean k is effectively independent of depth at ∼10-18.5±1 m² Consideration of the permeability values inferred from thermal modeling and metamorphic fluxes suggests a quasi-exponential decay of permeability with depth of log k ≈ -3.2 log z - 14, where k is in meters squared and z is in kilometers. At mid to lower crustal depths this curve lies just below the threshold value for significant advection of heat. Such conditions may represent an optimum for metamorphism, allowing the maximum transport of fluid and solute mass that is possible without advective cooling.