Each year, millions of visitors, foreign students, and immigrants come to the United States. Foreign visitors may enter on a legal temporary basis–that is, with an authorized period of admission that expires on a specific date–either (1) with temporary visas (generally for tourism, business, or work) or, in some cases, (2) as tourists or business visitors who are allowed to enter without visas. (The latter include Canadians and qualified visitors from 27 countries who enter under the visa waiver program.) The majority of visitors who are tracked depart on time, but others overstay–and since September 11, 2001, the question has arisen as to whether overstay issues might have an impact on domestic security. In this report, we (1) describe available data on the extent of overstaying, (2) report on weaknesses in the Department of Homeland Security’s long-standing overstay tracking system, and (3) provide some observations on the impact that tracking system weaknesses and significant levels of overstaying may have on domestic security.Significant numbers of foreign visitors overstay their authorized periods of admission. Based in part on its long-standing I-94 system for tracking arrivals and departures, the Department of Homeland Security (DHS) estimated the overstay population for January 2000 at 2.3 million. But this estimate (1) excludes an unknown number of long-term overstays from Mexico and Canada, and by definition (2) excludes short-term overstays from these and other countries. Because of unresolved weaknesses in DHS’s long-standing tracking system (e.g., noncollection of some departure forms), there is no accurate list of overstays. Tracking system weaknesses make it difficult to monitor potentially suspicious aliens who enter the country legally–and limit immigration control options. Post-September 11 operations identified thousands of overstays and other illegal immigrant workers who (despite limited background checks) had obtained critical infrastructure jobs and security badges with access to, for example, airport tarmacs and U.S. military bases. As of April 2004, federal investigators had arrested more than 1,360 illegal workers, while the majority had eluded apprehension. Together with other improvements, better information on overstays might contribute to a layered national defense that is better able to counter threats from foreign terrorists. A more comprehensive system, US-VISIT, the U.S. Visitor and Immigrant Status Indicator Technology, is being phased in. The design and implementation of US-VISIT, however, face a number of challenges. It is important that this new program avoid specific weaknesses associated with the long-standing system. Checking for these weaknesses might help identify difficult challenges in advance and–together with other efforts–enhance USVISIT’s chances for eventual success as a tracking system.