Mass transport through thermally activated molecular dispersal (diffusion) in multicomponent mixtures comprising membrane systems can exhibit complexities which require different levels of physical and theoretical descriptions of which we give examples. At the simplest level mean field level component flows are derived from gradients of chemical potential. We will focus primarily in our talk on the mean field description in homogeneous or inhomogeneous polymer systems. Penetrant diffusion in homogeneous “rubbery” polymer films or membranes can be relatively successfully described by a simple “free volume” theory. Inhomogeneities due to e.g. polymer crystallinity are a further complication and are an essential aspect in diffusion in very porous systems such as rigid polymer foams. Further complications arise in mechanically relaxing polymer films with memory which are not in mechanical equilibrium. Such effects as well as penetrant stress driven diffusion and dual mode sorption can arise in polymer penetrant systems below their glass transition temperature.