Due to a significant drop in the energy needed for RO desalination from 12 kWh/m(3) to-2 kWh/m(3) over the past 20 years, which is close to the theoretical minimum for recovering salt-free water from seawater of 1 kWh/m(3), the focus here is on the critical aspects that offer opportunities to further reduce costs. These include pre- and post-treatment; and analysis and optimization of the performance of RO systems, such as selectivity, capacity, and flux decline. Flux decline includes concentration polarization and fouling from inorganic, organic, and biological constituents. We call these the "three legs" that undergird all membrane processes. The approach here is quantitative and includes detailed fluid mechanics and associated mass transfer of RO systems for optimizing performance. (C) 2016 Published by Elsevier B.V.