Security cooperation with Europe has been the bedrock of American strategy for more than 50 years. Today, that relationship is undergoing both stress and refinement as Europe moves toward a more unified political and security identity, and as the United States responds to a changing global security environment. While many issues have the potential to complicate U.S.-European security cooperation, few are more pressing than the U.S. pursuit of missile defense and Europe's construction of European Security and Defense Policy (ESDP). The author explains the relationship between missile defense and ESDP and shows that, rather than serving as wedges between the United States and Europe, both of these can help construct a better security relationship. In fact, transatlantic cooperation and understanding of these issues is necessary for either of them to succeed.