Renewable energy is one of the four building blocks outlined in the EPA’s Clean Power Plan, and states are likely to rely upon the production of renewable energy as a part of their efforts to control carbon in the electric sector. Existing renewable energy certificate (REC) tracking systems ensure that renewable energy is metered, tracked, and accounted for across North America. This paper, by the Center for Resource Solutions and the Regulatory Assistance Project, explores how state air and energy regulators can utilize existing REC systems to ensure the proper accounting of renewable energy generation towards their carbon goals. These flexible systems were developed with stakeholder input, are already widely used and understood by generators and load-serving entities, and can help state regulators cost-effectively achieve emissions reductions under 111(d).