The stellar performance of BP's emission control program has led many observers, inside and outside BP, to ascribe success to the firm's emissions trading system. As countries and other firms have considered the adoption of trading systems they often point to BP's pioneering experience as a guiding star. Yet no study has ever explained the operation and impact of BP's trading system. Which factors truly drove the leaders of BP's business units to cut emissions? What lessons should be learned from BP's experience to guide other trading systems?