This chapter explores the role of the United Nations in environmental peacebuilding from a normative and an institutional perspective. First, key contributions in providing stronger protection for the environment and natural resources in connection to peacebuilding are outlined chronologically. These are discussed, focusing on critical unaddressed gaps, namely the tension between normative and operational mandates, and the issue of coordination in this interdisciplinary field. Second, to illustrate a successful integrated peacebuilding approach, the case of Sierra Leone is presented, focusing on how international law impacted the work of practitioners. Finally, the chapter shares five lessons learned on environmental peacebuilding.