According to the Department of Justice, the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) is the lead federal agency responsible for stopping the illegal flow of firearms, or gun trafficking, from the United States to Mexico. ATF has developed a nationwide strategy to reduce firearms trafficking and violent crime by seeking to prevent convicted felons, drug traffickers, and juvenile gang members from acquiring firearms from gun traffickers. These criminals often acquire firearms from persons who are otherwise not prohibited from possessing firearms, or by buying firearms from corrupt federal firearms licensees (FFLs) who sell firearms off-the-books in an attempt to escape federal regulation. ATF also reports that Mexican drug trafficking organizations (DTOs) are increasingly sending enforcers across the border to hire surrogates (straw purchasers) who buy several "military-style" firearms at a time from FFLs. The DTOs also reportedly favor pistols chambered to accommodate comparatively large cartridges that are capable of piercing through armor vests usually worn by law enforcement officers, and magazines capable of holding more than 10 rounds of ammunition. Less frequently, but no less troubling to law enforcement, the DTOs have also sought .50 caliber sniper rifles that are capable of penetrating bullet proof glass and lightly armored vehicles. ATF reports that there are around 6,700 FFLs in the United States operating in the Southwest border region of Texas, New Mexico, Arizona, and California. By inspecting the firearms transfer records that FFLs are required by law to maintain, ATF investigators are often able to trace crime guns from their domestic manufacturer or importer to the first retail dealer that sold those firearms to persons in the general public, generating vital leads in criminal investigations. In addition, by inspecting those records, ATF investigators sometimes discover evidence of illegal, off-the books transfers, straw purchases, and other patterns of suspicious behavior. During FY2006 and FY2007, ATF dedicated approximately 100 special agents (SAs) and 25 industry operations investigators (IOIs) to a Southwest border initiative known as "Project Gunrunner" to disrupt the illegal flow of guns from the United States into Mexico. By the end of FY2008, ATF had deployed 146 SAs and 68 IOIs to the Southwest border to bolster that initiative at a conservatively estimated cost of $32.2 million. The Omnibus Appropriations Act of 2009 included an increase of at least $5 million for Project Gunrunner, and the FY2009 Supplemental Appropriations Act includes an additional $14 million for this initiative. Both the House-passed and Senate-reported FY2010 Commerce, Justice, Science, and Related Agencies appropriations bill (H.R. 2847) would provide ATF with an $18 million increase for Project Gunrunner, an amount equal to the President's request. U.S. firearms laws currently govern the possession and transfer of firearms and provide penalties for the violation of such laws. "Gun trafficking," although not defined by statute, essentially includes the movement or diversion of firearms from legal to illegal markets. This report includes legal analyses of three ATF-investigated, Southwest border gun trafficking cases to illustrate the federal statutes that are typically violated as part of wider gun trafficking schemes. The report also examines anti-gun trafficking proposals introduced in the 110th Congress. So far, no similar proposals have been introduced in the 111th Congress. The report concludes with possible policy questions for Congress regarding the magnitude of Southwest border gun trafficking, the use and significance of ATF crime gun trace data, the possible ratification of an Inter-American Gun Trafficking Convention (CIFTA), and the adequacy of the federal statutes designed to deter and reduce illegal gun trafficking.