Aqueous diffusion coefficients have been experimentally determined in a variety of porous/fractured geologic and engineered media. For performance assessment applications, the purely diffusive flux must be separated from retardation effects. The simple diffusion coefficient, D, does not include any transient chemical effects, e.g., sorption, which lower the diffusion coefficient for some finite time period until equilibrium is reached. D is primarily a function of volumetric water content, θ, and not material characteristics. At high water contents, D gradually declines as water content decreases, from 10−5 cm2/sec at θ ∼ 50% to 10−7 cm2/sec at θ ∼ 5%, followed by a sharp decline to 10−10 cm2/sec at θ ∼ 0.5%. Although surface diffusion has a strong experimental basis in the transport of gases along metal surfaces, experimental evidence for aqueous geologic/backfill/engineered systems strongly indicates that surface diffusion is not important, even in bentonite, because of the extremely poor connectivity among electric double-layers and the extremely low diffusivities and high ∂C/∂x at small area/point contacts which more than negate the increased flux along intragrain surfaces.