Electrodialysis (ED) is a membrane desalination process that was initially developed in the 1950s. The process uses direct current power to remove salts and other ionized species through cation and anion ion selective membranes to a concentrate collecting stream. The membranes are ion exchange resins cast in sheet form. The selectivity for cations or anions is created by the negative or positive charge given to the ion exchange sites in the resin. In the early 1970s, electrodialysis reversal (EDR) was developed. EDR is a mechanical enhancement of the ED process where the polarity of the applied DC power is periodically reversed which exposes the membrane surfaces and flow compartments to concentrate and the desalting stream.This paper discusses the use of EDR to desalt surface and wastewaters and the four basic reasons why this process is successful which include: membrane tolerance to turbidity; membrane tolerance for continuous exposure to oxidizing disinfectants to prevent biological fouling; membrane resistance to irreversible fouling; and, the existence of effective methods to recover systems from physical and chemical fouling problems should they occur. Includes 3 references, tables, figures.