After a long period of dormancy, membrane separation processes have begun to emerge as technically significant and commercially relevant unit operations. Prior to the mid-sixties, synthetic membranes were employed for those few specialized laboratory applications which could tolerate low permeability and poor selectivity or in electrochemical applications excluding, e. g., batteries, fuel cells, chloride-alkali electrolysis, where marginal chemical stability remained a severe limitation. Within the framework of a broad R & D program started in the US in the mid-fifties and devoted to the production of fresh water from brackish and seawater, developments of more suitable membranes arose out of the application of the principles of physical chemistry, modern polymer chemistry (especially surface or interfacial polymerization and polycondensation technology), and electron microscopy. In particular, it was learned that asymmetric membrane structures comprise a very thin consolidated barrier layer (5000 Å or less for membranes with economically practical filtration rates) supported by an integral but less dense substrate which does not participate in the transport process. Later and after much effort, composite membranes were developed in which the salt-rejecting skin (still only 5000 Å thick) was placed atop a supporting matrix formed from a more chemically and mechanically stable polymer.—The main desalination research effort led to several spin-off developments in related membrane fields, e.g. the successful preparation and commercialization of ultrafiltration technology in the automobile, food, and chemical industries. Also, ion-exchange membranes prepared from perfluorinated polymers offered the electrochemical industry much better chemical stability than the earlier phenolic-resin-based ion-exchange membranes.—Current efforts are aimed at the improved selectivity and stability required for very specific separation processes (e.g. separation of heavy metal salts from waste water or selective enrichment of gases). In the future, the mechanisms of biological processes will have to be exploited for successful development of synthetic membranes suitable for more sophisticated separations.