Although thermal desalination technology provides potable water in arid regions (e.g., Israel and the Gulf), its relatively high cost has limited application to less arid regions with large populations (e.g., California). Energy-intensive distillation is currently being replaced with less costly pressure- and electrically driven membrane-based processes. Reverse osmosis (RO) is a preferred membrane technology owing to process and pre- and posttreatment improvements that have significantly reduced energy requirements and cost. Further technical advances will require a deeper understanding of the fundamental science underlying RO. This includes determining the mechanism for water selectivity; elucidating the behavior of molecular water near polar and apolar surfaces, as well as the advantages and limitations of hydrophobic versus hydrophilic pores; learning the rules of selective water transport from nature; and designing synthetic analogs for selective water transport. Molecular dynamics simulations supporting experiments will play an important role in advancing these efforts. Finally, future improvements in RO are limited by inherent technical mass transfer limitations.