The energy efficiency of Reverse Osmosis (RO) desalination is a vital aspect of its economic feasibility and environmental impact. Today, desalination plants are powered mostly by fossil fuels, resulting in a substantial carbon footprint. Efforts need to be directed toward improving energy efficiency, without unduly increasingly the capital cost or compromising salt rejection and water recovery. This chapter reviews the breakdown of energy consumption by the different stages of the desalination process, including pre- and post-treatment. The RO process itself consumes the most energy, significantly exceeding ideal minimum consumption. Energy losses occur in the membranes, pumps, and energy recovery devices. This chapter explores research and technology trends in each area. Innovative membrane designs, using new structures and materials to improve permeability and salt retention, are described. These include nanocomposite, biomimetic, and graphene-based membranes. Approaches to plant configuration are presented that increase energy efficiency while reducing capital cost and improving operational flexibility. These include concentrate and permeate staging, hybrid membrane arrangements and, the more recently developed time-varying pressure systems such as batch RO. Positive displacement feed pumps can provide superior efficiency over centrifugal pumps. The trend in energy recovery devices has been using isobaric devices with booster pumps.