The European Union has come a long way in developing a security and defence policy of its own the last decade. The focus of the current research project is the EU`s performance in NATO. The project aims at advancing our understanding of the impact of the EU in NATO by analysing how the EU influences NATO decisions - both when acting as an institution and when acting as individual member states. Assessing the "internal" and "external" effectiveness of the EU are important when measuring and analysing the E U´s present role and potential as a security actor in general and in NATO in particular. Whilte EU military and civilian capabilities and enhanced administrative capacities are important for its impact in NATO, the project also explores the EU effect on t he ideas and values that gide NATO´s decisions and military involvement. National interests are prevalent within security and defence, potentially blocking European unity in NATO. Competing views among central EU members like France, Great Britain and Ger many may hamper the EU´s performance in NATO. Although majority of the formal EU decisions about security and defence are still made by unanimity, the EU institutions have obtaines increased influence over the preparation and shaping of these decisions. P olicies developes at EU level increasingly sees to influence the individual security policy of European states, and even non-EU states. The project will apply concepts like "policy communicaties" and "multi-level governance" to capture the complecity of E U policy and of EU actorness in the security policy field. Varying membership status in NATO of EU-member states may also explain the EU´s performance in NATO, and forthcoming enlargement processes of both organisations could potentially play into this. T he EU´s influence on NATO policy also hinges on the transatlantic relationship, and on how the bilateral relations between the EU and the United States will develop under the current US administration.