This report examines the history of pay-for-performance (P4P) energy efficiency approaches. As the report describes, there is a diverse spectrum of pay-for-performance programs but, at the most basic level, these programs track and reward energy savings as they occur, usually by examining data from a building's energy meters -- as opposed to the more common approach of estimating savings in advance of installation and offering upfront rebates or incentives in a lump-sum payment. The report finds that P4P has some important opportunities for increasing energy savings, but also key limitations that will need to be better understood through piloting and experimentation.