Membrane processes are able to separate liquid mixtures as well as mixtures of gases or vapours according to the sizes, solubilities, rates of diffusion or charges of the components. A separation is also possible using membranes in which the transport is coupled to a specific chemical reaction between the permeand and the membrane (facilitated or carrier mediated transport). This paper discusses aspects related to carrier-mediated separation processes such as the transport mechanisms, the choice of appropriate carriers, the incorporation of carriers in the polymeric membrane material, the coating of porous support membranes with thin selective layers of the carrier-polymer and, finally, the choice of appropriate parameters for the separation process. The separation of gas mixtures by carrier-membranes appears very promising from a technical point of view. Experimental membrane samples often show excellent separation factors, but they have not met the permeability requirements for an economic use so far.