The NSA's mission, as set forth in Executive Order 12333, is to collect information that constitutes "foreign intelligence or counterintelligence" while not "acquiring information concerning the domestic activities of United States persons". The NSA has declared that it relies on the FBI to collect information on foreign intelligence activities within the borders of the USA, while confining its own activities within the USA to the embassies and missions of foreign nations.

The NSA's domestic surveillance activities are limited by the requirements imposed by the Fourth Amendment to the U.S. Constitution; however, these protections do not apply to non-U.S. persons located outside of U.S. borders, so the NSA's foreign surveillance efforts are subject to far fewer limitations under U.S. law. The specific requirements for domestic surveillance operations are contained in the Foreign Intelligence Surveillance Act of 1978 (FISA), which does not extend protection to non-U.S. citizens located outside of U.S. territory.

The activities described below, especially the publicly acknowledged and so-called 'Domestic Phone' tapping and Domestic Call Database programs, have prompted questions about the extent of the NSA's activities and concerns about privacy and the rule of law.

A wiretapping program named ThinThread was tested in the late 1990s, according to information obtained by the Baltimore Sun in 2006. This program may have contributed to the underlying technology used in later systems, but its safeguards on privacy were abandoned after the 9/11 attacks.

On December 16, 2005, the New York Times reported that, under White House pressure and with an executive order from President George W. Bush, the National Security Agency, in an attempt to thwart terrorism, had been conducting phone-taps on individuals in the U.S. calling persons outside the country, without obtaining warrants from a secret court as required by the Foreign Intelligence Surveillance Act (FISA).

Proponents of the warrantless surveillance claim that the President has the authority to order such action, arguing that the President has powers under the Constitution that trump laws such as FISA. In addition, some argued that FISA was implicitly overridden by a subsequent statute, the Authorization for Use of Military Force, although most concede this argument is untenable after the Supreme Court's ruling in Hamdan v. Rumsfeld. The only judge to rule on the matter thus far, in the case ACLU v. NSA, concluded that such surveillance is illegal and unconstitutional; her decision is stayed pending appeal. Third party legal authorities agree that the surveillance is illegal or probably illegal, although there is more disagreement as to whether it is unconstitutional. See NSA warrantless surveillance controversy for details.

In the years after President Nixon resigned, there were several investigations of suspected misuse of CIA and NSA facilities. Senator Frank Church headed a Senate investigating committee called the Church Committee which uncovered previously unknown activity, such as a plot to assassinate Fidel Castro by the CIA, which had been ordered by President John F. Kennedy and Attorney General Robert F. Kennedy. During the investigation, it was also found that the NSA was actively tapping the phones of targeted American citizens. After the Church Committee hearings, the Foreign Intelligence Surveillance Act of 1978 became law, limiting circumstances under which domestic surveillance was allowed.