Cornell University Legal Information Institute: Immigration
Federal immigration law determines whether a person is an alien, the rights, duties, and obligations associated with being an alien in the United States, and how aliens gain residence or citizenship within the United States. It also provides the means by which certain aliens can become legally naturalized citizens with full rights of citizenship. Immigration law serves as a gatekeeper for the nation’s border, determining who may enter, how long they may stay, and when they must leave.