"The unauthorized immigrant (illegal alien) population in the United States is a key and controversial immigration issue. Competing views on how to address this population have been, and continue to be, a major obstacle to enacting immigration reform legislation. 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 and other methods. These estimates can help inform possible policy options to address the unauthorized alien population. Both the Department of Homeland Security (DHS) and the Pew Research Center estimate that approximately 11.5 million unauthorized aliens were living in the United States in January 2011. DHS further estimates that there were some 11.4 unauthorized residents in January 2012. Pew has released a preliminary estimate of 11.7 million for the March 2012 unauthorized resident population. The Immigration and Nationality Act (INA) and other federal laws place various restrictions on unauthorized aliens. In general, they have no legal right to live or work in the United States and are subject to removal from the country. At the same time, the INA provides limited avenues for certain unauthorized aliens to obtain legal permanent residence."
CRS Report for Congress, R41207
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html