This article seeks to systematize the study of environmental cooperation and to define its scope more clearly, by identifying the conditions under which environmental cooperation best facilitates conflict transformation and peacebuilding, and which forms of negotiation or stakeholder participation have been particularly successful. I reviewed and evaluated past experiences experiences to pinpoint the lessons learned, as well as shortcomings, and I highlight areas for action and recommendations for development agencies. I analyzed not only the existing literature but also an exemplary selection of studies on water cooperation and cooperation in nature conservation in southern Africa, the Middle East, Latin America, and Central Asia.