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 often required to allow generated energy to be utilized effectively. This book reviews some of the important technologies for energy storage that utilize mechanical methods and thermal methods. In the first part of the book, mechanical energy storage methods are considered. These methods include pumped hydroelectric energy storage, gravitational potential of solid masses, and flywheels. In the second part, various methods of storing thermal energy are discussed. These include the use of the sensible heat of materials, the latent heat associated with phase transitions, and heat associated with chemical reactions.