This report presents estimates of the number of unauthorized immigrants in the United States in mid-2001 for the total unauthorized population, Mexican unauthorized population, and non-Mexican Central American unauthorized population. Researchers subtracted estimates of the numbers of persons residing in the country legally from the numbers of foreign-born persons in official government surveys (which contain both legal and unauthorized persons), then adjusted for extra undercount of such persons in the surveys. Overall, the size of the total unauthorized population in mid-2001 varied from 3.4-5.8 million, with a mid-range of 7.8 million. The size of the unauthorized non-Mexican Central American population varied from 1.2-1.9 million, with a mid-range of 1.5 million. For the total and Mexican unauthorized populations, researchers estimated the numbers of years that members of these populations had been in the U.S. Overall, 2.35 million unauthorized Mexicans would be eligible for legalization if at least 10 years of U.S. residence were required, with 3.5 million eligible if at least 5 years were required. About 3.8 million people in the total unauthorized population would be eligible if at least 10 years of residence were required, with 5.8 million eligible if at least 5 years were required. (Contains 11 references.) (SM)