Water heating represents significant energy use in residential applications. Therefore, energy efficient water heating is an essential component to achieve national energy saving goals. Although heat pump water heaters have been around for over three decades, this technology has only matured in recent years, in part because of improved systems and infrastructure as well as emerging energy conservation standards on water heating. The goal of this study is to provide a comprehensive and in-depth review of heat pump water heater research on system energy efficiency and performance topics. The study focuses on laboratory and field (in-situ) experiments and measurements, modeling of energy use and efficiency, technological modifications or upgrades, and control operation strategies; with emphasis on the Coefficient of Performance (COP), reliability, and energy savings. The review shows that while most of current heat pump water heater systems operate in the COP range of 1.8-2.5, there are some potential technological updates that could augment the current systems and increase COP to a range of 2.8-5.5. Issues that could dampen the adoption of these technologies are discussed. The review identifies key areas for future studies that are still lacking in support of various changes suggested for increasing heat pump water heaters efficiency and performance. (c) 2017 Elsevier B.V. All rights reserved.