In December 2000 the European Union formalized a common European Security and Defense Policy (ESDP) with treaty amendments and concrete measures to establish supporting political and military structures. The United States favors a greater role for the European Union (EU) in European security provided that NATO's role in transatlantic security is not weakened and that European Union efforts result in increased and relevant military capabilities. Given the nascent nature of the ESDP, the time is now to carefully analyze the challenges and opportunities that exist for the United States to actively participate in shaping ESDP and NATO-EU relations. The United States seeks a relationship with the EU that strengthens transatlantic security and leads to an EU which is capable of being a future, viable partner for global security. How can the United States influence the process to ensure evolving EU defense structures are complementary and interoperable with NATO and capable of sharing defense responsibility with the United States in the future?