In recent years, both compliance and voluntary markets have emerged to help support the development of renewable energy resources. Compliance markets are primarily driven by state renewable portfolio standards (RPS), which require utilities or other load serving entities to procure renewable energy for a portion of their electricity supplies. Voluntary markets differ in that they provide consumers with the option to purchase or support renewable energy for a portion or all of their electricity needs. We refer to this as the voluntary market or the green power market because these renewable energy purchases are made on a voluntary basis, driven largely by an interest in using cleaner and more sustainable sources of electricity. Both of these markets are growing rapidly. Today, about half of U.S. states have RPS policies in place, with a number of these policies adopted in the last several years.