This paper examines the experience of the United Nations Environment Programme (UNEP), specifically, its Post-Conflict Assessment Unit (PCAU), as an instrument of post-conflict environmental peacemaking. The ecological challenges facing war-torn societies can be as daunting as the social ones, as people struggle to provide themselves with clean water, sanitation, food and energy supplies in settings marked by toxification, the destruction of infrastructure, the loss of livelihoods, and the disruption of local institutions. Over time, these immediate environmental difficulties evolve into more diffuse but equally important challenges: establishing systems of environmental governance, creating the requisite administrative and institutional capacity, and finding sustainable trajectories for economic reconstruction and development. If handled effectively, environmental challenges may create a solid foundation for peace and sustainable development; if handled poorly, they risk undercutting the already tenuous peace that typically marks such situations. The paper identifies common patterns across several recent cases in which UNEP/PCAU has been active. Particular attention is paid to UNEP’s impact, or lack thereof, with regard to four core themes: (1) the problem of reconstituting government; (2) monitoring and assessment challenges in militarized environments; (3) linking environmental assistance and humanitarian aid; and (4) moving from environmental clean-up to planning and administration.