Both solar photovoltaics and wind energy have variable and uncertain outputs, which are unlike the dispatchable sources used for the majority of electricity generation in the United States. The variability of these sources has led to concerns regarding the reliability of an electric grid, as well as the cost of integrating large amounts of variable generation into the electric grid. This book explores the role of energy storage in the electricity grid, focusing on the effects of large-scale deployment of variable renewable sources, primarily wind and solar energy.