Demand response refers to the intentional modification of electricity usage by end-use customers during system imbalances or in response to market prices. While initially developed to help support electric system reliability during peak load hours, demand response resources currently provide an array of additional services that help support electric system reliability in many regions of the United States. These same resources also promote overall economic efficiency, particularly in regions that have wholesale electricity markets. Recent technical innovations have made it possible to expand the services offered by demand response and offer the potential for further improvements in the efficient, reliable delivery of electricity to end-use customers. This report reviews the performance of demand response resources in the United States, the program and market designs that support these resources, and the challenges that must be addressed in order to improve the ability of demand response to supply valuable grid services in the future.