The Department of Homeland Security estimates that in January 2009 there were 10.8 million illegal immigrants in the United States, down from 11.6 million in January 2008. The estimated peak of illegal immigrants was 11.8 million in January 2007. Thus, the total number decreased by one million between 2007 and 2009. Go here for a complete report by DHS, which estimates the number of country of origin and by state of residence in the U.S.