During the last 35 years membranes have evolved from a laboratory tool to industrial products with significant technical and commercial impact. Today, membranes are used for desalination of sea and brackish water and for treating industrial effluents. They are efficient tools for the concentration and purification of food and pharmaceutical products and the production of base chemicals. Furthermore, membranes are key components in artificial organs, drug delivery devices, and energy conversion systems. In combination with conventional techniques membranes often provide cleaner and more energy-efficient production routes for high-quality products. Fundamental aspects of membranes and membrane processes are discussed, as well as technically and economically effective applications. The present and future membrane market is assessed and new developments and future research needs are discussed.