This project examined the state Renewable Portfolio Standard (RPS) and its diffusion as a policy instrument to promote the use of renewable energy. Through state case studies, logit analysis, and ordinal regression, the political process of adoption is examined. External and internal policy diffusion determinants were examined for their possible contributions to the state adoption or non-adoption of the RPS policies. In comparison with earlier diffusion studies, including those on RPSs, adjacent state adoption does not play a major role in RPS adoption into 2010, though other elements of the external diffusion model do appear relevant. Both the case studies and the quantitative analyses found internal determinants to be important. A general economic and quality of life index proved to be a significant predictor of RPS adoption. However, the studies consistently found that the states' environmental-political liberalism and renewable energy resource bases were the predominant factors affecting adoption of both any RPS program and the more advanced or upgraded RPS programs.