The Foreign Intelligence Surveillance Act (FISA) is a law enacted in 1978 that sets out procedures for physical and electronic surveillance and collection of foreign intelligence information. FISA was created in response to congressional investigations into federal surveillance activities conducted in the name of national security, and it sought to provide judicial and congressional oversight of foreign intelligence surveillance activities while maintaining the secrecy necessary to effectively monitor national security threats.

FISA regulates a number of the most sensitive types of intelligence collection activities that occur within the United States. It establishes procedures for the authorization of electronic surveillance, use of pen registers and trap and trace devices, physical searches, and business records for the purpose of gathering foreign intelligence. FISA also established the United States Foreign Intelligence Surveillance Court (FISC), a special U.S. Federal court that holds nonpublic sessions to consider issuing search warrants under FISA.

The law requires federal law enforcement and intelligence agencies to obtain authorization for gathering "foreign intelligence information" between "foreign powers" and "agents of foreign powers" suspected of espionage or terrorism. Although FISA was initially limited to government use of electronic surveillance, subsequent amendments have broadened the law to regulate other intelligence-gathering methods, including physical searches, pen register and trap and trace (PR/TT) devices, and compelling the production of certain types of business records.

FISA has been repeatedly amended since the September 11 attacks, with several added provisions garnering political and public controversy due to privacy concerns. The law has been expanded and broadened to make it a useful tool in exposing and combating foreign terrorist groups’ efforts to target the United States.