"As the 111th Congress debates comprehensive immigration reform and its component parts of immigration control (i.e., border security and interior enforcement), legal reform (i.e., temporary and permanent admissions), and the resolution of unauthorized alien residents, concerns arise over the capacity of the Department of Homeland Security to identify and remove temporary aliens who fail to depart after their visas expire. It is estimated that each year hundreds of thousands of foreign nationals overstay their nonimmigrant visas or enter the country illegally (with fraudulent documents or bypassing immigration inspections). The most recent published estimate based upon the March Supplement of the Current Population Survey (CPS) is that 11.9 million unauthorized aliens were residing in the United States in 2008. Reliable estimates of the number of nonimmigrant overstays are not available, and the most recent sample estimates range from 31% to 57% of the unauthorized population (depending on methodology) in 2006. This report will be updated."
CRS Report for Congress, RS22446