Four of the September 11 hijackers who entered the United States with legal visas overstayed their authorized periods of admission. This has heightened attention to issues such as the extent of overstaying, weaknesses in the current overstay tracking system, and how the tracking system weaknesses and the level of overstaying might affect domestic security. The GAO study found that significant numbers of foreign visitors overstay their authorized periods of admission. The DHS estimates the resident overstay population at 2.3 million as of January 2000. Because the starting point for this estimate is the 2000 census, it does not cover short-term overstays who have not established residence. It also omits potential long-term overstays of persons who have entered the country from Mexico and Canada. Weaknesses in the DHS's current system for tracking visitors' arrivals and departures include failure to collect some departure forms and the inability to match other departure forms to arrivals. Because of the inadequacies of the current tracking system, there is no accurate list of overstays. New tracking initiatives intended to address these weaknesses include the National Security Entry and Exit Registration System, which does not, however, cover most visitors; and the U.S. Visitor and Immigrant Status Indicator Technology, a more comprehensive, automated program. Like other illegal immigrants, potential terrorists who overstay authorized periods of admission obtain jobs with fraudulent identity documents, including jobs at critical infrastructure locations. Improved tracking of overstays is a critical element of an effective, comprehensive domestic security system. Testimony before the Subcommittee on Immigration, Border Security, and Claims, Committee on the Judiciary, House of Representatives. 2 figures and appended supplementary data on visitor arrivals and overstays