Has the emergence of new transnational threats—terrorism, drug cartels, natural disasters—affected the dynamics of security relations among Canada, Mexico, and the United States? What is the likely future of these relations in a highly securitized world? Richard Kilroy, Abelardo Rodríguez Sumano, and Todd Hataley trace the evolution of security relations in North America from the bilateral approach that existed prior to the events of September 11, to the unilateral US approach of perimeter defense after September 11, to the creation of a trilateral regional security framework. Their analysis highlights both the move toward cooperation and the significant obstacles that limit the potential for an effective regional security complex.