The Future of Energy Storage study is the ninthin the MIT Energy Initiative's Future of series,which aims to shed light on a range of complexand vital issues involving energy and the envi-ronment.Previous studies have focused on therole of technologies such as nuclear power,solar energy, natural gas, geothermal, and coal(with capture and sequestration of carbondioxide emissions), as well as systems suchas the U.S. electric power grid.Central to allthese studies is understanding the role theseparticular technologies can play in both decar-bonizing global energy systems and meetingfuture energy needs. Energy storage will playan important role in achieving both goals bycomplementing variable renewable energy(VRE) sources such as solar and wind,which are central in the decarbonizationof the power sector.