The United States Constitution creates a government of enumerated powers. The Constitution delegates to the United States Congress the power “[t]o establish an uniform Rule of Naturalization, … throughout the United States.” Thus, the Constitution gives Congress the power to determine which foreigners can become citizens, and under what conditions. The Constitution, however, is silent on immigration. That is, the federal government is not explicitly granted a general power to exclude or remove noncitizens from the United States.

Nevertheless, U.S. courts have allowed the federal government to exercise such a power. At various points in time (and in various U.S. Supreme Court cases), this power over immigration has been said to derive from various legal standings: the Commerce Clause, the Naturalization Clause, the Declaration of War Clause, the Law of Nations Clause, and the Necessary and Proper Clause, among others.

One clause that would appear to at least imply a power over immigration is the Migration and Importation Clause. That clause provides that “[t]he Migration or Importation of such Persons as any of the States now existing shall think proper to admit, shall not be prohibited by the Congress prior to the year one thousand eight hundred and eight.” It is argued that, it now being after 1808, Congress has the power to prohibit migration or importation of persons. Despite the use of the word “migration,” this clause is now interpreted to relate to slavery and the slave trade, not immigration.

As early as 1798, with the Alien and Sedition Acts, and with the United States on the brink of war with France, Congress acted on immigration. It allowed the President to imprison or deport immigrants, at the time deemed "aliens," who were considered dangerous to “peace and safety” (Alien Friends Act of 1798) or were citizens of a nation with which the United States was at war (Enemy Alien Act of 1798). It also increased the residency requirement for naturalization from five to 14 years (Naturalization Act of 1798).

Still, in the first nearly hundred years following the ratification of the U.S. Constitution, the source of Congress’ power to act generally on immigration was moot, since immigrants were allowed in without restriction. However, in 1875, Congress barred the immigration of convicts and prostitutes. In 1882, Congress forbade immigration of people with limited resources and people with mental health conditions, categorized at the time as "paupers" and the “mentally defective.”

Also in 1882, with the Chinese Exclusion Act, Congress prohibited immigration by Chinese people. It was in litigation over this act that the Supreme Court began to articulate a basis for federal power to act over immigration. In that and subsequent cases, the Supreme Court has held that the power to exclude immigrants is a function, not of a power explicitly set forth in the Constitution, but an “incident of sovereignty.”

In Chae Chan Ping v. United States, 130 U.S. 581 (1889), the Supreme Court stated:

"That the government of the United States, through the action of the legislative department, can exclude aliens from its territory is a proposition we do not think open to controversy. Jurisdiction over its own territory to that extent is an incident of every independent nation. It is a part of its independence.

"If it could not exclude aliens, it would be to that extent subject to the control of another power …"

In Fong Yue Ting v. United States, 149 U.S. 698 (1893), the Supreme Court further held that the ability to deport immigrants was likewise a “right of a nation.”

"The right of a nation to expel or deport foreigners who have not been naturalized, or taken any steps towards becoming citizens of the country, rests upon the same grounds, and is as absolute and unqualified as the right to prohibit and prevent their entrance into the country."

This is an exclusively federal power, the Supreme Court has said; the states may not regulate in the area of immigration:

"The Federal Government has broad constitutional powers in determining what aliens shall be admitted to the United States, the period they may remain, regulation of their conduct before naturalization, and the terms and conditions of their naturalization … Under the Constitution, the states are granted no such powers; they can neither add to nor take from the conditions lawfully imposed by Congress upon admission, naturalization and residence of aliens in the United States or the several states."

The nature of the congressional power over immigration, the Supreme Court has held, is “plenary.” That is, the decisions of Congress concerning immigration are relatively free of review by the courts. Still, the Supreme Court has indicated that the plenary power is subject to constitutional limits.

Since the 1880s, Congress has exercised its plenary power extensively. In the 1920s, immigration was based on a national origin quota system. The number of immigrants allowed was limited based on how many immigrants from a nation were already in the U.S., according to past U.S. census figures. As a consequence, immigration was relatively higher from Northern and Western Europe than other areas.

Today, the federal power over immigration is set forth in the Immigration and Naturalization Act. As originally enacted in 1952, it maintained certain exclusions and altered the previous rules set by the national origin quota.

Amendments in 1965 eliminated national origin quotas, instead emphasizing family reunification. That is, the immigration system prioritized immigration of family members of those already in the United States. Subsequent amendments in 1986, 1990, and 1996 granted partial amnesty to illegal immigrants, increased levels of immigration, established diversity programs, and increased penalties for violations of immigration law. The Patriot Act of 2001 toughened background checks for admittance to the United States.

U.S. federal policy on immigration has changed frequently over time, with some policies favoring open admissions and others favoring tighter restrictions. Some of this has been attributed to the unclear constitutional basis for federal authority in this area—since the Constitution does not explicitly set forth who is in charge, each branch of government has, at times, attempted to assert its authority.