The total capacity of operating and projected desalination plants in Saudi Arabia is 725.4 mgd (2.74 Mm(3)/d). Reverse osmosis (RO) only accounts for about 10% of the total desalted capacity. However, there is a recent trend toward the use of more RO in seawater desalination either for new plants or in connection with the present multi-stage flash (MSF) plants. MSF is preferred for large-scale dual-purpose seawater desalination plants. For single-purpose plants RO becomes a competitive choice, especially for moderate capacity. Power generation, desalting capacity and fuel cost strongly influence the selection of the desalination process. Process schemes also play an important role in this regard. MSF plants coupled with gas or steam turbines, RO with or without energy recovery and RO/MSF hybrid are examples of possible process schemes. In Saudi Arabia the first MSF plant was started in Al-wajh and Duba in 1928 with a capacity of 60,000 gpd (227.1 m(3)/d). The first seawater RO plant in Saudi Arabia was installed in 1979. Recently numbers of RO and MSF plants were in production in different locations in Saudi Arabia. A discussion of the advantages and disadvantages of RO and MSF processes will be given as well as a detailed comparison between them, Plant production and capacity and energy consumption will be considered with reference to Saudi Arabia.