Immigration policy has been an ongoing subject of congressional attention in recent years and a topic of concern for the U.S. public at large. Mexicans are by far the largest group of U.S. migrants, and about 1 in 10 Mexicans now live (legally or illegally) in the United States. Indeed, Mexico-U.S. migration represents the largest binational migration flow in the world. What does Mexico's prominence in the U.S. migration system mean for U.S. immigration policy? On one hand, it means that U.S. immigration policy, to varying degrees, primarily affects Mexicans and Mexico. Today's Mexico-U.S. migration flows and the Mexico-born population in the United States are the product of previous immigration policy decisions, as well as of the long and complex history of the U.S. and Mexican economies, labor markets, and demographics. On the other hand, it also means that Mexico remains at the center of today's immigration debate, even if sometimes only implicitly. Recognizing Mexico's status within the U.S. migration system focuses attention on how the U.S. immigration debate affects Mexico, and on how Mexico may affect certain migration outcomes. This report begins with an overview of Mexico-U.S. migration flows, and reviews the history of migration policies in both countries. The report then describes current demographics of Mexico-born persons in the United States and their recent dispersion to new U.S. destinations—factors that have helped shape the politics of U.S. immigration policy in recent years. The last section of the report discusses four major issues in the U.S. immigration debate: migration control and border security, the lawful permanent resident (LPR) visa system, temporary worker programs, and potential legalization programs for certain unauthorized aliens. For each of these issues, the report describes the impact of proposed reforms on Mexicans and raises questions about how Mexico may affect policy outcomes. Does Mexico support U.S. immigration policy goals? Should the United States and Mexico pursue more collaborative approaches to certain immigration issues? The report also examines a fifth, related policy issue: efforts to reduce emigration pressures within Mexico. Migration to the United States consists of three main groups of migrants: LPRs, temporary nonimmigrants, and unauthorized aliens. Within each of these categories, Mexicans represent the largest group of foreign born in the United States. Lawful permanent residents are foreign nationals who live lawfully and permanently in the United States, and they are typically eligible to apply for U.S. citizenship five years after receiving their visas. The Immigration and Nationality Act (INA) specifies a complex set of numerical limits and preference categories for permanent immigration reflecting the principles of family reunification, the admission of immigrants with needed skills, the protection of refugees, and diversity by country of origin. The INA prioritizes family-based immigration, making more than three times as many visas available in the family-based preference categories as in the employment-based categories. The INA does not set aside LPR visas for Mexico, but Mexicans are especially likely to take advantage of the law's family-friendly rules, with 122,686 Mexicans becoming LPRs as immediate relatives of U.S. citizens or family-sponsored immigrants in FY2010 (see Figure 1 ). Overall, 88% of Mexicans were admitted in one of the family categories in 2010, compared to 67% of all LPRs. The figures differ even more for the decade 2000-2009: 93% of Mexicans were family-based compared to 65% of all LPRs (see Appendix B ). Foreign nationals who are admitted to the United States for a temporary period of time and an expressed reason are known as nonimmigrants. There are 24 major nonimmigrant visa categories, commonly referred to by the letter and numeral that denotes their subsection in Section 101(a)(15) of the INA, including for example B-2 tourists, E-2 treaty investors, and F-1 foreign students. The nonimmigrant visa categories authorizing employment include the H-2A visa for agricultural guest workers, the H-2B visa for other lower-skilled seasonal or intermittent workers, the H-1B visa for temporary professional workers, the J-1 cultural exchange visa, the E visa for treaty traders and treaty investors, and the L visa for intra-company transferees. Temporary professional workers from Canada and Mexico also may enter under terms set by the North American Free Trade Agreement (NAFTA) on NAFTA professional (TN) visas. Mexico was the top sending county of temporary nonimmigrants in FY2010, making up 27.8% of all such entries. Aside from tourists and business visitors, the large majority of Mexican nonimmigrants enter as H-2A or H-2B low-skilled workers (see Figure 2 ). Mexico was one of 58 countries eligible to send H-2A and H-2B nonimmigrants (as of January 2012), and Mexicans accounted for 82.9% of such low-skilled nonimmigrant visas issued in 2010. Mexicans represent a small proportion of other legal nonimmigrants (also see Appendix C ). About 11.4 million unauthorized aliens from various countries were estimated to be in the United States in 2010, down from about 12.1 million in 2007. Between one-half and two-thirds of unauthorized aliens enter without inspection (by crossing the border between ports of entry or being smuggled through a port) or enter illegally by using fraudulent documents. The remainder enter legally as nonimmigrants but then remain past the visa expiration date (becoming visa overstayers) or otherwise violate the terms of their nonimmigrant visa. Of the 11.4 million, an estimated 6.7 million unauthorized Mexicans resided in the United States in 2010, meaning about 59% of the unauthorized population was from Mexico (see " Legal Status "). In recent years, the George W. Bush and Barack Obama Administrations, along with some Members of Congress, have favored "comprehensive immigration reform" (CIR) packages that would include reforms to the LPR and nonimmigrant visa systems to expand legal inflows, legalization for certain unauthorized aliens, and new migration control measures. Congress has considered a number of CIR bills and related proposals during this period, but none have been signed into law. Thus, legislative and administrative action during the last decade mainly has focused on new enforcement measures at the U.S.-Mexican border and within the United States; and a record number of unauthorized aliens have been removed in each year since 2003, with Mexicans accounting for almost three-quarters of all removals (see " Immigration Enforcement and Border Security "). This section describes how social, economic, and demographic factors in Mexico and the United States along with migration-related policies in both countries have produced four phases in the regional migration system: limited seasonal flows prior to World War II, the Bracero temporary worker program from 1942 to 1964, the emergence of a predominantly illegal system from 1965 through the 1980s, and the consolidation of that system along with increased family-based immigration since the 1990s. While seasonal migration by Mexican agricultural workers dates back to the 19 th century—and while most of California, Arizona, New Mexico, Texas, Nevada, Colorado, and Utah once belonged to Spain and later Mexico—large-scale permanent immigration from Mexico to the United States is a recent phenomenon, as Figure 3 illustrates. Mexican LPR immigrants exceeded 1 million in a decade for the first time in the 1980s (the darker bars in Figure 3 ) and Mexicans accounted for more than 15% of total LPR inflows for the first time in the 1990s (the line in Figure 3 ). The lighter bars in Figure 3 depict U.S. census data on the total Mexican-born population living in the United States, which include legal immigrants, temporary nonimmigrants, and unauthorized migrants. As the figure illustrates, fewer than 1 million Mexicans lived in the United States as recently as 1970. All of these Mexican migration and population trends increased markedly beginning around the 1970s, however, with legal immigration (LPR inflows) and the total Mexico-born population living in the United States both roughly doubling during each decade beginning in 1970. Even as growth rates slowed after 2000, the Mexico-born population in the United States climbed to 11.5 million people by 2009. In contrast with earlier periods, in which the migration system was dominated by short-term, seasonal, migration for agricultural work in the U.S. Southwest, Mexican migrants in the United States today are equally divided by gender; most live permanently in the United States; they work overwhelmingly in non-agricultural occupations (see Table 3 ); and they are dispersed throughout the entire country (see Figure 6 ). What explains the shift from relatively low immigration levels for the first two-thirds of the 20 th century to the rapid growth of Mexican migration since 1970? International migration is primarily a function of structural demographic, economic, and social forces. High birth rates and limited economic opportunities have been "push factors" that encouraged emigration from Mexico for most of this period; and plentiful employment opportunities, connections to family networks, and an aging population have been "pulls factors" within the United States—though all of these push and pull factors are in flux. How these structural forces translate into migration outcomes also depends on migration-related policies in both countries that encourage or discourage international migration and that afford aliens different legal statuses. Push and pull factors propelling Mexico-U.S. migration were relatively limited in the early 20 th century. Transportation and social networks linking Mexicans to the United States were poorly developed, and migrant farm workers had limited access to labor markets beyond the four southwest border states.