Desalination by reverse osmosis (RO), which first entered commercial use in the 1970s, was initially mainly used for treating brackish water. Technological progress led to the development of an RO membrane enabling single-pass seawater desalination. Toyobo succeeded in developing a single-pass seawater desalination RO module composed of hollow fiber type membranes made of cellulose triacetate in 1978, and then in 1979 began production of the first commercially available double-element module. This double-element module has many advantages suitable for seawater desalination. It has high chlorine tolerance and high salt rejection, derived from the properties of the membrane material, and it is highly resistant to fouling and scaling matters due to the unique flow pattern and fiber bundle configuration. These advantages help to explain why the Toyobo double-element module has been used so successfully at the many seawater desalination plants around the world. Since the 1980s, large plants capable of desalinating several tens of thousands of cubic meters a day have sprung up around the Mediterranean and In the Middle East. The Jeddah RO Phase I Plant, which has a capacity of 56, 800m$^3$/day, went into operation in 1989. In 1994, the same sized Phase II Plant came on stream, giving the plant a huge total capacity of 113, 600m$^3$/day. The plant constructor Mitsubishi Heavy Industries, Ltd. (MHI), and the RO membrane manufacturer Toyobo Co., Ltd. In 1998, the world's largest RO seawater desalination plant in operation, which has a capacity of 128, 000m$^3$/day and is run by Saudi Arabia's Saline Water Conversion Corporation (SWCC), went into operation at Yanbu. RO seawater desalination technology has thus already reached the stage of full-scale commercial use. In order to encourage its wider use, however, RO desalination needs to be made more economical by lowering construction and water treatment costs. Toyobo has therefore developed a new economical RO desalination system by a recovery ratio of 60% using a high-pressure module with a high product flow rate. In 2000, Toyobo high recovery membrane module was selected for the largest seawater desalination plant in Japan, which has a capacity of 50, 000m$^3$/day.