"The Foreign Intelligence Surveillance Act (FISA) was enacted in 1978. This legislation was the Congressional response to the exposure during multiple Committee hearings of previous abuses of U.S. persons' privacy rights by certain components of the United States government. Those abuses had occurred, according to the government, as part of its efforts to counter purported threats to national security. Unquestionably, such threats existed in and before 1978; beyond peradventure, however, they pale in comparison to the threats to national security that the United States currently faces. Those threats bear the face of terrorism, primarily foreign but domestic as well. Though FISA is not a legally usable tool for combating domestic terrorism, its electronic surveillance and physical search authority are legal and very effective methods for monitoring the activities of foreign powers and agents of foreign powers while they operate within the United States. Increasingly, indeed, overwhelmingly, the current objective of such operational activities is to thwart terrorist acts. [...] In the face of the foregoing, it is no wonder then that our Congress has recently enhanced the ability of counter terrorism agents to use FISA in the effort to discover and thereby thwart acts of terrorism aimed at our country. The following is offered as an historical context in which to view FISA as well as a summary of how FISA has evolved over the past quarter century of its existence as a tool not just to thwart terrorism but more generally as an effective means to gather foreign intelligence."
Federal Law Enforcement Training Center: http://www.fletc.gov