Reverse osmosis is a highly effective separation technology used industrially in several areas, including desalination of seawater and brackish water and wastewater treatment. Membrane materials and types of modules used in reverse osmosis, as well as theoretical bases and operational aspects, are presented. A discussion of flux limiting conditions caused by concentration polarization and membrane fouling is included, as is that of the pretreatments required for process feed streams. The application of reverse osmosis for the separation of hazardous organic compounds from aqueous systems, for wastewater treatment, desalination, ultrapure water production, and food processing are reviewed in terms of membrane performance. The emerging applications involving solvent resistant membranes, and functionalized membranes are also included. The concept of nanofiltration and its applications are also discussed. General design considerations and economics are reviewed.