The number of foreign-born people residing in the United States (an estimated 40 million) is at the highest level in our history and, as a portion of the U.S. population, has reached a percentage (12.9%) not seen since the early 20 th century. The actual number of unauthorized aliens in the United States is unknown. The three main components of the unauthorized resident alien population are (1) aliens who overstay their nonimmigrant visas, (2) aliens who enter the country surreptitiously without inspection, and (3) aliens who are admitted on the basis of fraudulent documents. In all three instances, the aliens are in violation of the Immigration and Nationality Act (INA) and subject to removal. The last major law that allowed unauthorized aliens living in the United States to legalize their status was the Immigration Reform and Control Act (IRCA) of 1986 ( P.L. 99-603 ). Generally, legislation such as IRCA is referred to as an "amnesty" or a legalization program because it provides LPR status to aliens who are otherwise residing illegally in the United States. Among IRCA's main provisions was a time-limited legalization program, codified at Section 245A of the Immigration and Nationality Act, that enabled certain illegal aliens who entered the United States before January 1, 1982, to become LPRs. It also had a provision that permitted aliens working illegally as "special agricultural workers" to become LPRs. Nearly 2.7 million aliens established legal status through the provisions of IRCA. How to address the estimated 11.7 million unauthorized aliens residing in the United States is perhaps the most controversial issue in the comprehensive immigration reform (CIR) debate. Reducing the population of unauthorized aliens living in the United States is a common goal, but disagreement about how to do so remains. Some proponents of CIR favor large-scale legalization proposals that would enable most unauthorized residents to become LPRs, if they meet specified conditions and terms as well as pay penalty fees. Some proposals to provide immigration relief to unauthorized aliens would target only particular subsets of this population, such as those with pathways to LPR status under current law (e.g., those with U.S. citizen or LPR family members or employers who have petitioned for them), or unauthorized aliens who were brought to the United States as children. Others counter that Congress should not consider immigration relief or legalization until current laws are more reliably enforced. They also maintain that it would be unfair to reward unauthorized migrants at the expense of potential immigrants who are waiting to come legally and that legalization would serve as a magnet for future unauthorized migrant flows. This CRS report presents data estimating since 1986 the number of unauthorized aliens who have been living in the United States. There have been a variety of estimates of the unauthorized resident alien population over this period, sometimes with substantially different results. This report is limited to analyses of the Current Population Survey (CPS) conducted by the U.S. Census Bureau and the Bureau of Labor Statistics and of the American Community Survey (ACS) conducted by the U.S. Census Bureau so that there are basic standards of comparison over time. Because the CPS and the ACS are both sample surveys of the U.S. population, the results are estimates. Additionally, while the data distinguish between the foreign born who have naturalized and those who have not, they do not identify immigration status (e.g., legal permanent resident, refugee, temporary foreign worker, foreign student, unauthorized alien). Summaries of the detailed analyses of the March CPS, the ACS, and the monthly CPS are presented separately because each of these surveys is based on different questions and sample sizes. The most recent estimate (published in 2013) is that 11.7 million foreign nationals resided in the United States without authorization in 2012. For a basis of comparison, Figure 1 presents the estimate of 3.2 million unauthorized resident aliens in 1986 calculated by demographers Karen Woodrow and Jeffrey Passel, who worked for the U.S. Census Bureau at that time. As expected after the passage of IRCA, the estimate for 1988 dropped to 1.9 million. During the first decade after IRCA, researchers projected that the net growth in unauthorized aliens had averaged about 500,000 annually; analyses done during the early 2000s estimated the average growth at 700,000 to 800,000 unauthorized alien residents annually. By 2002, the estimated number of unauthorized resident aliens had risen to 9.3 million. Appendix B provides further discussion. The estimated number of unauthorized alien residents peaked in 2007, when Passel estimated that there were 12.4 million unauthorized alien residents in the United States ( Figure 1 ). The confidence intervals around the 12.4 million estimate ranged from a low of 11.9 million to a high of 12.9 million. "Thus, average annual growth over the 5-year period since 2000 was more than 500,000 per year. This number reflects the number of new unauthorized migrants arriving minus those who either die, return to their country of origin, or gain legal status." Michael Hoefer, Nancy Rytina, and Bryan C. Baker of the Department of Homeland Security's (DHS's) Office of Immigration Statistics (OIS) published their 2007 estimates of the unauthorized resident alien population from the American Community Survey (ACS) of the U.S. Census Bureau and yielded results consistent with Passel's estimates. An estimated 11.8 million unauthorized immigrants were living in the United States in January 2007 compared to 8.5 million in 2000 (Hoefer, Rytina, and Campbell, 2006). Between 2000 and 2007, the unauthorized population increased 3.3 million; the annual average increase during this period was 470,000. Nearly 4.2 million (35 percent) of the total 11.8 million unauthorized residents in 2007 had entered in 2000 or later. An estimated 7.0 million (59 percent) were from Mexico. The OIS reported an estimated 11.6 million unauthorized alien residents as of January 2008. This estimate marked the first decrease since the OIS began producing annual estimates in 2005. "The decrease in the size of the unauthorized population between 2007 and 2008," the OIS demographers pointed out, "may be due to sampling error in the estimate of the foreign-born population in the 2007 ACS." They estimated that the unauthorized alien population in the United States increased by 37% over the previous eight years. Drawing on a different release of the CPS and a slightly different methodology, Steven Camarota and Karen Jensenius of the Center for Immigration Studies estimated that the unauthorized resident alien population fell to 10.8 million in February 2009. They used tabulations from the monthly CPS data rather than the March supplement to calculate a decrease of 1.7 million (13.7%) in 2009 from their high of 12.5 million unauthorized resident aliens in 2007. Despite the differences in methodology and data, these conclusions were comparable to Hoefer, Rytina, and Baker and to Passel and D'Vera Cohn. Subsequently, Camarota used a combined two-year sample of the CPS (March 2010 and 2011) to estimate 10.5 million unauthorized resident aliens. He pointed out: It must also be remembered that these figures are only for those in the CPS, not those missed by the survey. Estimates prepared by other researchers often adjust for undercount in Census Bureau data. While there is debate about the number missed, most research indicates that roughly 10 percent of illegals are not counted in Census Bureau surveys such as the CPS. Thus, the true size of the illegal population could be 11.5 million. By 2010, most published estimates showed a continued decline in the number of the unauthorized alien residents. Analyses of both the CPS and the ACS data indicated that the estimated number dropped in 2008 and in 2009 before leveling off in 2010. According to Passel and Cohn, "The decline in the population of unauthorized immigrants from its peak in 2007 appears due mainly to a decrease in the number from Mexico, which went down to 6.5 million in 2010 from 7 million in 2007." In 2012, OIS updated their initial 2010 unauthorized estimates, which used Census 2000 population weights updated for births, deaths, and internal and international migration, from 10.8 million to 11.6 million. The revised 2010 estimates are only for the total unauthorized population. OIS indicated that they made other changes, most notably improving the 2011 estimates of the legally resident foreign-born population with the addition of approximately 100,000 LPRs who were not originally included in the annual LPR flow data between 2001 and 2010 because of late or delayed record keying. Hoefer, Rytina, and Baker wrote, "(C)onsidering that the revised 2010 estimate—which is based on more recent data—is higher than the reported estimate, it is possible that the DHS unauthorized estimates for 2005-2009 also underestimate the unauthorized immigrant population." Despite their upward revision in estimates, they concluded: "it is unlikely that the unauthorized immigrant population increased thereafter given relatively high U.S. unemployment, improved economic conditions in Mexico, record low numbers of apprehensions of unauthorized immigrants at U.S. borders, and greater levels of border enforcement." Similarly, Passel, Cohn, and Ana Gonzalez-Barrera of the Pew Research Center revised their estimates in 2013 to conform to the 2010 census. Rather than showing a decrease of 1.3 million from 2007 to 2009, the new estimates show a decrease of 0.1 million from 2007 to 2009. Their new estimates indicate that the decline in authorized resident aliens after 2007 has stalled in 2012, and they suggest the trend may be reversing. Appendix A presents a table detailing their revised estimates. The annual March CPS provides detailed socioeconomic data that are not available in the monthly CPS, shedding further light on the traits and trends in authorized migration.