This review by the Department of Justice (Department) Office of the Inspector General (OIG) examined the impact of the Bureau of Alcohol, Tobacco, Firearms and Explosives' (ATF) implementation of Project Gunrunner on the illicit trafficking of guns from the United States to Mexico. Violence associated with organized crime and drug trafficking in Mexico is widespread, resulting in tens of thousands of deaths. In part because Mexican law severely restricts gun ownership, drug traffickers have turned to the United States as a primary source of weapons, and these drug traffickers routinely smuggle guns from the United States into Mexico. The criminal organizations responsible for smuggling guns to Mexico are typically also involved in other criminal enterprises, such as drug trafficking, human trafficking, and cash smuggling. This requires ATF to work with other federal entities, as well as with state and local law enforcement partners, in sharing intelligence, coordinating law enforcement activities, and building cases that can be prosecuted. To help combat firearms trafficking into Mexico, ATF began Project Gunrunner as a pilot project in Laredo, Texas, in 2005 and expanded it as a national initiative in 2006. Project Gunrunner is also part of the Department's broader Southwest Border Initiative, which seeks to reduce cross-border drug and firearms trafficking and the high level of violence associated with these activities on both sides of the border. In June 2007, ATF published a strategy document, Southwest Border Initiative: Project Gunrunner (Gunrunner strategy), outlining four key components to Project Gunrunner: the expansion of gun tracing in Mexico, international coordination, domestic activities, and intelligence. In implementing Project Gunrunner, ATF has focused resources in its four Southwest border field divisions. In addition, ATF has made firearms trafficking to Mexico a top ATF priority nationwide. The OIG conducted this review to evaluate the effectiveness of ATF's implementation of Project Gunrunner. Our review examined ATF's enforcement and regulatory programs related to the Southwest border and Mexico, ATF's effectiveness in developing and sharing firearms trafficking intelligence and information, the number and prosecutorial outcomes of ATF's Project Gunrunner investigations, ATF's coordination with U.S. and Mexican law enforcement partners, ATF's traces of Mexican "crime guns," and challenges that ATF faces in coordinating efforts to combat firearms trafficking with Mexico.