Membranes are widely used for separation of ions from a mixture in aqueous solution. Major applications include removal of sulfate from seawater, removal of salt from cheese whey, and separation of sodium chloride from seawater for manufacture of table salt and brine for soda industry. The conventional reverse osmosis membranes are not capable of selectively separating electrolytes from the aqueous mixture. The charged nanofiltration membranes that have been developed over the last decade are now being used for some of these applications. This review is based on the theories of transport of solute and water through these membranes and the mechanism of permselectivity. The application of these theories to estimate the membrane design parameters, i.e., pore size, porosity, charge density and thickness has been discussed. The capabilities of different nanofiltration membranes have been illustrated with examples and compared with reverse osmosis membranes. The information provided here is expected to provide an extensive information on the subject.