Foreign Intelligence Surveillance Act (FISA): An Overview [Updated April 6, 2021] [open pdf - 441KB]
From the Introduction: "Congress enacted the Foreign Intelligence Surveillance Act (FISA) in 1978. FISA provides a statutory framework for government agencies to obtain authorization to gather foreign intelligence by means of (1) electronic surveillance, (2) physical searches, (3) pen registers and trap and trace (PR/TT) devices (which record or decode dialing, routing, addressing, or signaling information), or (4) the production of certain business records. Agencies typically request authorization for such activities from the Foreign Intelligence Surveillance Court (FISC), a specialized court created by FISA to act as a neutral arbiter of agency requests for FISA orders."
CRS In Focus, IF11451
Congressional Research Service: https://crsreports.congress.gov/