This report from the U.S. Department of Energy examines the impact of Renewable Energy Certificates (RECs) on the development of renewable energy sources. To promote the use of renewable power, some states mandate that renewable power plants issue an REC (with a specific serial number) for each 1 MW of power generated. RECs may be sold to utilities and consumers that cannot access the actual power, but want to contribute to the development of renewable power sources. Some RECs are issued on a voluntary basis. The impact of RECs on the development of renewable energy sources is analyzed and methods for encouraging such projects are discussed.