All countries have some form of immigration law to regulate the movement of persons across international borders, and to define the rights and obligations of foreign nationals.

In the United States, immigration laws address the following concerns, among others:

    which persons are eligible to lawfully enter the country;

    what are the terms and conditions under which foreign nationals can either visit, or live, work, or conduct business in the United States;

    what are the consequences of violating one's immigration status; and

    what are the means whereby persons may acquire, retain, or lose the rights of citizenship.

Foreign nationals seeking to enter the United States, or seeking to lawfully remain in the United States, or seeking to become lawfully present after a period of unlawful presence, must navigate a highly complex network of regulations and procedures.

To legally enter the United States, a noncitizen must have either a visa, or some legal substitute for a visa. For instance, a visitor must either have a visitor visa, or be eligible to have that requirement waived. A refugee must either enter after a grant of refugee status, or qualify for asylum after entry. Immigration statuses, as such, generally entail some connection to a public purpose the government seeks to promote, such as tourism, trade, industry, education, health care, family unity, or the protection of persecuted people.

Legal grounds of inadmissibility apply to some persons seeking to legally enter the United States, and grounds of deportability apply to certain foreign nationals who have violated the terms of their lawful admission to the United States.

The immigration laws also determine which persons are citizens of the United States at birth, and the means whereby certain persons who are not citizens at birth may later become citizens.