Creating an effective and integrated national homeland security effort is a significant challenge. Europe and the United States have reacted differently to the emergence of mass casualty terrorism, but must work together to cope with the diverse issue areas, sectors, professions, and relevant actors involved in such a broad-based concept. The authors suggest that Europe and the US have a lot to gain by coordinating more closely, and that the exchange of experience is crucial as we attempt to stay ahead of a learning enemy.