The last time any group crunched the numbers, around 40 to 45 percent of those living in the U.S. illegally overstayed their visas.
That was according to the Pew Research Center back in 2006 but most policy makers accept that number as being pretty consistent and continue to cite it today.
As a result, the Department of Homeland Security is trying to tackle the issue with one of its most ambitious efforts to date.
Starting this week, it will begin taking scans of the faces and eyes of all foreigners entering the U.S. on foot at San Diego's Otay Mesa.
If it's successful, the program will be rolled out across the almost 2,000 mile stretch of the U.S.–Mexico border.
Marc Rosenblum, deputy director of the U.S. Immigration Policy Program at the Migration Policy Institute, joins Take Two to explain how it works.