There is virtually an infinite amount of water in the oceans, but because of an average salt content of 3.5 g/mL, it is essentially unusable for any beneficial use to humans without significant treatment. Membrane separation processes have come to the forefront in the last three decades and remain the principle method of purifying salt water. This chapter summarizes a wide range of membrane separation process topics by addressing membrane separation principles, reverse osmosis, ultrafiltration, microfiltration, gas permeation, pervaporation, and electrodialysis. An application section provides three Illustrative Examples related to the general subject of membrane separation and membrane separation processes.