Reverse osmosis has come of age as a unit operation and is finding increasing usage in water and waste treatment. Large-scale (100–1000 gpm) units are currently operating throughout the world on a wide variety of feed streams. This paper summarizes the status of development of reverse-osmosis processing of municipal and industrial waste streams. Plants are described which process secondary effluent, metal-finishing wastes, and textile dye wastes. Capital and operating costs are also presented for reverse osmosis and compared with those of other desalting methods.