The European Security and Defence Policy (ESDP) is one of the most widely discussed areas of EU activity. Since its inception, the EU has used this new instrument to deploy missions to all parts of the world in a flurry of diplomatic, civilian, and military activity. This article examines the ESDP by discussing, in turn, its origins, development, accomplishments, and, finally, the attempts made by scholars to apply theoretical insights to these. It argues that the Union has, to date, failed to live up to the hopes inspired by the creation of the ESDP. Equally, however, a burgeoning theoretical literature about it holds out the prospect of many interesting theoretical debates to come.