Immigration to the United States is a complex demographic phenomenon that has been a major source of population growth and cultural change throughout much of the history of the United States. The economic, social, and political aspects of immigration have caused controversy regarding ethnicity, economic benefits, jobs for non-immigrants, settlement patterns, impact on upward social mobility, crime, and voting behavior. In 2005, the United States per capita ranked 34th out of 179 world nations in the number of immigrants allowed into the country.^[1]

Prior to 1965, policies such as the national origins formula limited immigration and naturalization opportunities for people from areas outside Western Europe. Exclusion laws enacted as early as the 1880s generally prohibited or severely restricted immigration from Asia, and quota laws enacted in the 1920s curtailed Eastern European immigration.

The civil rights movement of the 1960s led to the replacement^[2] of these ethnic quotas with per-country limits.^[3] Since then, the number of first-generation immigrants living in the United States has quadrupled,^[4] from 9.6 million in 1970 to about 38 million in 2007.^[5] Nearly 14 million immigrants entered the United States from 2000 to 2010,^[6] and over one million persons were naturalized as U.S. citizens in 2008. The per-country limit^[3] applies the same maximum on the number of visas to all countries regardless of their population and has therefore had the effect of significantly restricting immigration of persons born in populous nations such as Mexico, China, India, and the Philippines – the leading countries of origin for legally admitted immigrants to the United States in 2013;^[7] nevertheless, China, India, and Mexico were the leading countries of origin for immigrants overall to the United States in 2013, regardless of legal status, according to a U.S. Census Bureau study.^[8] As of 2009, 66% of legal immigrants were admitted on the basis of family ties, along with 13% admitted for their employment skills and 17% for humanitarian reasons.^[9]

For those who enter the US illegally across the Mexico–United States border and elsewhere, migration is difficult, expensive and dangerous.^[10] Virtually all undocumented immigrants have no avenues for legal entry to the United States due to the restrictive legal limits on green cards, and lack of immigrant visas for low-skilled workers.^[11] Participants in debates on immigration in the early twenty-first century called for increasing enforcement of existing laws governing illegal immigration to the United States, building a barrier along some or all of the 2,000-mile (3,200 km) Mexico-U.S. border, or creating a new guest worker program. Through much of 2006 the country and Congress was immersed in a debate about these proposals. As of April 2010 few of these proposals had become law, though a partial border fence had been approved and subsequently canceled.^[12]