This thesis contributes to three debates: 1) the debate over whether the UN and regional security actors actually support each other or come into conflict; 2) the debate regarding which conditions are conducive for cooperation between international organisations; and 3) the debate over 'the coordination challenge' in peacebuilding. The thesis aims to test the liberal assumption that the EU and UN peacebuilding objectives are aligned and that they simply need to link up better to meet them. Its central hypothesis is that even though the EU and UN share values and broad objectives, organisational interests in cooperation are not necessarily aligned because of inter-institutional competition for resources, and intra-institutional competition related to how to address the challenges of peacebuilding. Qualitative analysis of policy evolution shows that EU and UN development actors share a similar transformative approach to peacebuilding that seeks to address risk factors for conflict, while security actors share a crisis management approach predicated on exerting leverage over national elites in line with short-term priorities. Quantitative and qualitative empirical analysis of operational cooperation reveals that EU and UN development actors have an increasingly strong operational relationship and that they share material and normative interests in cooperation. In contrast, the relationship between EU and UN security actors is more competitive. Nevertheless, under conditions of UN overstretch and relative weakness, the UN has a material interest in operational cooperation, while the EU interest in cooperation is principally normative. This has led to symbolic forms of institutionalised cooperation, notably the EU-UN Steering Committee on Crisis Management and the UN Peacebuilding Commission. Empirical study of these institutions confirms that they satisfy organisational interests in symbolic forms of cooperation, but have little impact on the material dynamics of cooperation and are not appropriately configured to deliver cross-sectoral or inter-institutional coherence.