By analyzing the UN Environment Programme’s (UNEP’s) contribution to peacebuilding, this book aims to show how international bureaucracies develop knowledge and thereby come to matter on the world stage. Portraying UNEP as an open system, it explores how a growing understanding within the Programme of how environmental degradation shapes insecurities and vice versa has motivated its work on peacebuilding. The theoretical part of this book addresses knowledge, open systems, and knowledge creation. It then presents a historical discussion of UNEP’s development in an open system context. Finally, it investigates how knowledge emergence on the linkage between the environment, conflicts, and insecurities influenced UNEP’s interests and its work on environmental peacebuilding.