This paper examines law enforcement and security context of the U.S.-Mexican border region and the new challenges that have developed since September 11 as a result of new terrorist concerns. The authors explore the conventional understanding of U.S.-Mexican relations and the question of whether there is a “security community” along the border. The authors map the law enforcement and security structures that are of significance in shaping the U.S.-Mexican relationship, particularly the new Department of Homeland Security. The authors highlight successful instances of U.S.-Mexican collaboration in the San Diego-Tijuana region as possible models for best practices in other parts of the U.S.-Mexican border region.