A well-known characteristic of clay-rocks and engineered bentonite barriers is their low hydraulic permeability. In porous media with such low k values, at hydraulic pressure gradients representative of subsurface conditions, molecular diffusion is an important mass transport mechanism on length scales up to hundreds of meters and millions of years. Because of the important role of molecular diffusion in clay materials, extensive experimental efforts have aimed to characterize the associated diffusion coefficients. Characterization of these diffusion coefficients is nontrivial, because of the need to surmount experimental challenges associated with the large swelling pressure and fine-grained nature of clay minerals and to account for theoretical complexities, such as the couplings between adsorption, diffusion, and mechanics at multiple scales. The present chapter summarizes the available experimental database on diffusion and briefly discusses its implications regarding the fundamental phenomena associated with diffusion in clay-rocks and engineered clay barriers. It is shown that the relationships between microstructure and the macroscopic parameters for diffusion processes remain poorly constrained. This is one of the reasons for the existence of many different conceptual models in the literature. In addition, little agreement exists regarding fundamental phenomena that occur within individual pores, such as the structure of the electrostatic double layer and the mobility of water and ions near clay mineral surfaces. Future studies should focus particularly on determining the relative importance of short-range (surface complexes) and long-range (diffuse ion swarm) interactions in screening the negative surface charge density of clay minerals, and the mobility of water and ions as a function of distance from clay mineral surfaces. Additional efforts should also be focused on developing models that can describe diffusion results in both bentonite and clay-rocks, and that are based on the same underlying concepts.