Energy use is not uniform throughout the day and year. Renewable energy resources such as hydroelectric, wind, and solar energy, generate different amounts of energy over time and space, and are not always available when and where they are needed. A storage mechanism is required to allow generated energy to be utilized effectively. This book considers some of the important technologies for energy storage beginning with batteries, which utilize electrochemical reactions to store electrical energy, with applications that range from consumer electronics to transportation and grid-scale storage. Next, the storage of energy in electric and magnetic fields through the use of supercapacitors and superconducting magnets, is discussed. Finally, chemical approaches to energy storage such as hydrogen and electrofuels are discussed.