"The unauthorized alien (illegal alien) population in the United States is a key and controversial immigration issue. In recent years, competing views on how to address this population have proved to be a major obstacle to enacting comprehensive immigration reform legislation. The unauthorized alien issue is likely to be a key challenge if, as the Senate Majority Leader and the Speaker of the House have indicated, the 111th Congress takes up immigration reform legislation this year. It is unknown, at any point in time, how many unauthorized aliens are in the United States, what countries they are from, when they came to the United States, where they are living, and what their demographic, family, and other characteristics are. Demographers develop estimates about unauthorized aliens using available survey data on the U.S. foreign-born population. These estimates can help inform possible policy options to address the unauthorized alien population. According to recent estimates by the Department of Homeland Security (DHS), approximately 10.8 million unauthorized aliens were living in the United States in January 2009. Using different sources, the Pew Hispanic Center has estimated the March 2008 unauthorized resident population at about 11.9 million."
CRS Report for Congress, R41207
U.S. Department of State: http://www.state.gov