Nowadays membrane filtration processes are used industrially as an alternative to conventional separation methods. Membrane separation methods can be divided into classes according to their separation characteristics: (i) separation by sieving action; (ii) separation due to a difference in affinity and diffusivity; (iii) separation due to a difference in charge of molecules; (iv) carrier-facilitated transport, and (v) the process of (time-) controlled released by diffusion. In all these cases diffusion processes play an important role in the transport mechanism of the solutes. Various mechanisms have been distinguished to describe the transport in membranes: transport through bulk material (dense membranes), Knudsen diffusion in narrow pores, viscous flow in wide pores or surface diffusion along pore walls. In practice, the transport can be a result of more than only one of these mechanisms. For all of these mechanisms models have been derived. The characteristics of a membrane, e.g. its crystallinity or its charge, can also have major consequences for the rate of diffusion in the membrane, and hence for the flux obtained. Apart from the diffusion transport processes in membranes mentioned above, other important diffusion processes are related to membrane processes, viz. diffusion in the boundary layer near the membrane (concentration polarization phenomena) and diffusion during membrane formation. The degree of concentration polarization is related to the magnitude of the mass transfer coefficient which, in turn, is influenced by the diffusion coefficient. The effect of concentration polarization can be rather different for the various membrane processes. The phase inversion membrane formation mechanism is determined to a large extent by the kinetic aspects during membrane formation, which are diffusion of solvent and of non-solvent and the kinetics of the phase separation itself.