Legal Standards for the Intelligence Community in Conducting Electronic Surveillance [open pdf - 36KB]
From the text: "(U) Electronic surveillance is conducted by elements of the Intelligence Community for foreign intelligence and foreign counterintelligence purposes. Because of its potential intrusiveness and the implications for the privacy of United States persons, such surveillance is subject to strict regulation by statute and Executive Order and close scrutiny. The applicable legal standards for the collection, retention, or dissemination of information concerning U.S. persons reflect a careful balancing between the needs of the government for such intelligence and the protection of the rights of U.S. persons, consistent with the reasonableness standard of the Fourth Amendment, as determined by factual circumstance."
Federation of American Scientists: http://www.fas.org/