The world increasingly depends on desalting seawater or brackish water for producing suitable and sustainable supplies of potable water for local populations, tourism, agriculture and industry. The energy cost component of desalinating seawater has historically been a large factor (up to 70%) of the total cost. There is a limit to the amount of available energy and an environmental consequence associated with every kilowatt consumed. Along with the older style centrifugal energy recovery devices (ERDs), there has been a recent proliferation of ERDs that employs positive displacement mechanisms. These "pressure-equalizing" or isobaric ERDs transfer the energy from the membrane reject stream directly to the membrane feed stream. This direct, positive displacement approach results in a net transfer efficiency of up to 97%. This efficiency advantage makes it possible to dramatically improve the performance of seawater reverse osmosis (SWRO) plants by reducing their energy consumption by as much as 60% compared to systems operating without energy recovery. In addition to energy savings, isobaric ERDs offer significant benefits to SWRO plant designers and operators. These include unlimited capacity, reduced high-pressure pump costs, high efficiency and operational flexibility. The PX pressure exchanger isobaric ERD provides the additional benefits of maintenance-free operation, fail-safe operation, corrosion avoidance, low vibration, ease of control and long life. Although the author of this paper is directly associated with Energy Recovery, Inc., a leading company in isobaric ERD technology, the principles and theories presented in this paper are applicable to all devices that are based on the positive displacement, isobaric chamber approach.