In recent years, membrane separation processes have evolved from simple applications in the laboratory to utilization in important industrial operations with significant technica] and commercial impact. Over the last two decades, considerable effort by physicists, chemists and engineers has been devoted to develop better membranes and extend their range of application. In this paper, the principles of various separation processes such as microfiltration, ultrafiltration, reverse osmosis, electrodialysis, and gas separations are described and a review of their present applications in the laboratory and in plant operations is given. The structure and transport properties of commercial membranes and their use in various applications are discussed in detail. More recent developments in membrane separation processes such as the use of liquid membranes and carrier facilitated transport for the selective removal of certain chemical species are also reviewed