The term "perimeter defense" has come back into vogue recently, with regard to security strategies for North America. The United States' concern primarily with the terrorist threat to its homeland subsequent to September 11, 2001 (9/11) is generating this discussion with its immediate neighbors of Mexico and Canada (and to some extent some Caribbean nations—the "third border"). The concept is simply that by pushing defenses out to the "perimeter" nations, then security will be enhanced, since the United States visions itself as more vulnerable to international terrorism than its neighbors. However, Canada and Mexico have not been very happy about the perimeter defined by Washington since 9/11. These nations have sought to define the trilateral relationship beyond just discussions of terrorism to include natural disasters and international organized crime as a component of a broader trilateral agenda. Eight years later these three nations continue to look for some convergence of security interests, although there remains a degree of tension and hesitancy towards achieving a "common security agenda" in the Western Hemisphere.This article examines the concept of "perimeter defense" within the context of the new security challenges that the United States, Mexico, and Canada face today. Questions to be addressed in the article include: Do all these nations share the same "threat" perception? Where exactly is the "perimeter?" What security arrangements have been tried in the past? What are the prospects for the future for increased security cooperation? The main focus of this article is at the sub-regional level in North America and whether a new "trilateral" strategic security relationship between the United States, Canada, and Mexico can emerge in North America.