Federal immigration law determines whether a person is an alien, and associated legal rights, duties, and obligations of aliens in the United States. It also provides means by which certain aliens can become naturalized citizens with full rights of citizenship. Immigration law serves as a gatekeeper for the nation's border: it determines who may enter, how long they may stay and when they must leave.

The United States has a long history of immigration laws. The Immigration and Nationality Act of 1952 (INA) with some major, and many minor, changes continues to be the basic immigration law of the country. The most significant ammendment to the INA was in 1965 which abolished the natural origin provisions, and established a new quota system.

For INA purposes, an "alien" is any person who is not a citizen or a national of the United States. There are different categories of aliens: resident and nonresident, immigrant and nonimmigrant, documented and undocumented ("illegal" ).

States have limited legislative authority regarding immigration, and 28 U.S.C. § 1251 details the full extent of state jurisdiction. Generally, 28 U.S.C. § 994 details the federal sentencing guidelines for illegal entry into the country.

Congress has total and complete authority over immigration. Power of the President is limited to policies on refugees. Unless the issue concerns the rights of aliens to constitutional protections the courts have rarely intruded.