This article by Bradford Roberts discusses making electricity grids "smarter" and modernizing them so that they can accept large amounts of renewable energy resources are fairly universally accepted as steps necessary to achieve a clean and secure electric power industry. The best way to achieve this goal is a topic of debate among power system designers. Although energy storage in utility grids has existed for many decades, the impact of storage in future grids is receiving more attention than ever from system designers, grid operations and regulators. The amount of storage in a grid and its value is also a subject of debate. Understanding the leading storage technologies and how they can affect grid operations is an important first step in this assessment.