The 'Berlin Plus' agreement of 16 December 2002 now allows the EU to draw on some of NATO'smilitary assets in its own peacekeeping operations.The exact legal nature of the agreement, however, appears doubtful. Yet - given that political actors often resort to legal arguments in case of a dispute - the question of whether it is binding under international law is likely to be raised in the future. First, a short historical overview is given of the development of relations between the EU and NATO. In some respects, these are a mirror image of the co-operation NATO previously entertained with the Western European Union. However, since the French-British meeting of St-Mâlo these relations have assumed features of their own, for instance equal status for the EU. The text of the 'Berlin Plus' agreement is then tested against the constitutive characteristics of a treaty in international law. In conclusion, the 'Berlin Plus'is nothing but a non-binding agreement.The most important reason for this is that the EU manifestly lacked treaty-making competence, as the agreement was concluded by its High Representative for the Common Foreign and Security Policy, not by the statutory organ provided for in such cases in the Treaty on European Union. Nevertheless, as the parties proceed to implement the 'Berlin Plus' according to its stated terms, legally binding force may arise for some of its contents through estoppel.