Millions live among us: Do they hurt or help?

How many immigrants in the country are illegal? Depending on the source, about 7 million to as many as 20 million.

Nailing down such figures is impossible. Even settling on a ballpark figure is difficult given the official sources: the U.S. Census, apprehensions along the U.S.-Mexico border, and social-service agencies. Based on the 2000 census, the Census Bureau estimates the number at 8.7 million. As of 2003, the Citizenship and Immigration Services put the number at 7 million. Immigration officials since have said the number has grown by as much as 500,000 a year.

Those closest to the fight to protect U.S. borders say the figure is higher. The Border Patrol union Local 2544 in Tucson, Ariz., estimates between 12 million and 15 million.

The Pew Hispanic Center, a nonpartisan research organization in Washington, D.C., estimates 11.5 million to 12 million "unauthorized migrants" live in the United States. Pew bases its numbers on the "Current Population Survey," a monthly assessment of about 50,000 households jointly conducted by the Bureau of Labor Statistics and Census Bureau. "Unauthorized migrants" include some who have temporary permission to live in the U.S. and those whose status is unresolved.

Citing school enrollments, foreign remittances, border crossings and housing permits, Bear Stearns Asset Management researchers estimate the number at as many as 20 million. Bear Stearns also found that illegal immigrants are sending home significant amounts of money, suggesting their numbers are considerably higher than official estimates.

One variable involves relatives who join those coming across the border and form larger family units. Nearly 14 million people (including 4.7 million children) live in "mixed-status" families, in which the head of the household or spouse is in the United States illegally, Pew reported last year. That is partly because U.S.-born children — regardless of their parents' legal status — are automatically citizens.

Whatever the total, the annual number of illegal immigrants has exceeded those coming legally for at least 10 years: 700,000 illegally on average compared with 610,000 legally, according to Pew.

The number of "unauthorized migrants" also has grown since legalization programs began in the mid-1980s, Pew reported in April: about 180,000 a year in the 1980s; 400,000 per year from 1990 to 1994; 575,000 per year from 1995 to 1999; and 850,000 per year from 2000 to 2005.