A design method for reverse osmosis desalination plants has been developed. It incorporates rigorous mathematical models for the prediction of the performance of various process units (reverse osmosis modules, pumps, energy recovery turbines) employed in the flowsheet and takes into account the network structure using an appropriate superstructure, which represents various reverse osmosis networks. Cost equations relating the capital and operating cost to the design variables, as well as the structural variables of the designed network have been introduced in the objective function. The total cost of the plant is minimized in order to determine the optimal operating and structural variables. The model is accurate enough to describe the process and yet simple enough to be used for design purposes. During the simulation and optimization studies, several structures for multistage reverse osmosis systems have been found. Results concerning the economics of the process are presented. Optimal results have also been used for the derivation of design curves concerning the effect of quality and quantity of produced water to the total annualized cost of the plant for various types of membrane modules.