The Immigration and Nationality Act of 1952 (INA), also known as the McCarran-Walter Act, eliminated all race-based quotas, replacing them with purely nationality-based quotas. The INA continues to influence the field of American immigration law. To enforce the quotas, the INA created the Immigration and Naturalization Service (INS). The INS served as the federal agency that enforced these caps for remainder of the 20th century.

When Congress passed the INA, it defined an "alien" as any person lacking citizenship or status as a national of the United States. Different categories of aliens include resident and nonresident, immigrant and nonimmigrant, and documented and undocumented ("illegal"). The terms "documented" and "undocumented" refer to whether an arriving alien has the proper records and identification for admission into the U.S. Having the proper records and identification typically requires the alien to possess a valid, unexpired passport and either a visa, border crossing identification card, permanent resident card, or a reentry permit. The INA expressly refuses stowaway aliens entry into the U.S.

The need to curtail illegal immigration prompted Congress to enact the Immigration Reform and Control Act (IRCA) of 1986. The IRCA toughened criminal sanctions for employers who hired illegal aliens, denied illegal aliens federally funded welfare benefits, and legitimized some aliens through an amnesty program. The Immigration Marriage Fraud Amendments of 1986 sought to limit the practice of marrying to obtain citizenship. The Immigration Act of 1990 thoroughly revamped the INA by equalizing the allocation of visas across foreign nations, eliminating archaic rules, and encouraging worldwide immigration.

The Illegal Immigration Reform and Immigrant Responsibility Act (IIRIRA) of 1996 revolutionized the process of alien entry into the United States. The IIRIRA eliminated the term "entry," replacing it with "admission." An application for admission occurs whenever an alien arrives in the U.S. regardless of whether the arrival occurs at a designated port-of-entry. Applicants at either designated ports or otherwise must submit to an inspection by U.S. customs, even if the applicant possesses an immigrant visa. The IIRIRA also employs the term "arriving alien" to describe applicant aliens attempting to enter the U.S., regardless of whether they arrive at a designated port, a non-designated point on the border, or are located in U.S. waters and brought to shore.