Energy storage is central for the entire grid, improving resources from wind, solar and hydro to nuclear and fossil fuels, to demand side resources and system efficiency benefits. Energy storage can be performed as a generation, transmission, or distribution asset, and times in a single asset. Energy storage is an enabling technology. When the sun isn't shining or the wind isn't blowing, energy storage can support. When demand shifts and baseload resources can't react quickly enough, again energy storage can support. It saves consumer cost, improves reliability and resilience, integrates generation sources, and helps reduce environmental impacts. This book discusses these aspects while comprehensively covering several energy storage technologies in operation and the ones under demonstration and development. Numerous references are cited for the reader to hunt for more details and if interested in research further. It serves as a text/reference book for students and as a manual for those in the industry and for policy makers. About the Energy Storage: United Nation's Secretary-general António Gueterres, speaking at the launch of the World Meteorological Organization's state of the global climate report on 18th may 2022, said: “First, renewable energy technologies, such as battery storage, must be treated as essential and freely available global public goods. Removing obstacles to knowledge sharing and technological transfer is crucial for a rapid and fair renewable energy transition. Storing renewable electricity is often cited as the greatest barrier to the clean energy transition. I am therefore calling for a global coalition on battery storage to fast-track innovation and deployment – a coalition led and driven by governments, bringing together tech companies, manufacturers, and financiers.” “VRE resources such as wind and solar depend on daily and seasonal variations and weather fluctuations; they aren't always available to be dispatched to follow electricity demand……. Energy storage can help VRE-dominated electricity systems balance electricity supply and demand while maintaining reliability in a cost-effective manner….” Says MIT Energy Initiative Director Robert Armstrong, Chevron Professor of Chemical Engineering and chair of the Future of Energy Storage study. The MIT Energy Initiative report confirms that energy storage makes deep decarbonization of reliable electric power systems affordable. “There is no better solution to achieve energy security at present other than the solar and wind energy solutions, coupled with storage systems such as batteries and pumps…….” Says Ajay Mathur, Director General of International Solar Alliance.