This is the second in a series of technical briefs exploring emerging opportunities for energy efficiency in residential and commercial buildings. The ability to store energy and use it later is growing increasingly important to commercial buildings, especially as renewable energy and other distributed resources are integrated into the grid. Buildings can use batteries that store electricity, or thermal energy storage systems that store chilled water, ice, or heat. This brief outlines the features and benefits of energy storage, defines some current challenges to widespread adoption, and describes several energy efficiency programs that incentivize these technologies. We conclude with recommendations to program administrators and policymakers to encourage the use of storage systems.