This paper examines cooperation between NATO and the European Union in defense and security issues. Over the last decade transatlantic relations demonstrates new dynamics because of the fast development of the EU Common security and defense policy (CSDP). This process led to confusion on both sides of the Atlantic and raise the question: Will the European Union and NATO compete or cooperate, and if so, what might it look like?

Key words: European Union, NATO, transatlantic relations, Common security policy and defenses (CSDP).