According to the Fourth Amendment, “The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated[.]” But how does that provision apply to the vast troves of information that Americans and businesses increasingly collect, send, receive, and store? -- On June 5, 2017, the Supreme Court granted certiorari in Carpenter v. United States to resolve the question of whether the Fourth Amendment prohibits warrantless gathering of historical cellular phone records that include location information, also known as historical cell-site location information (CSLI). Judge Raymond M. Kethledge wrote for a panel of the Sixth Circuit that “although the content of personal communications is private, the information necessary to get those communications from point A to point B is not.” Historical CSLI, Judge Kethledge wrote, “fall[s] on the unprotected side of [the] line” because it is routing information used to “facilitate personal communications, rather than part of the content of those communications themselves.” Thus, the government does not need a warrant to obtain the information. -- The doctrine applied by the Sixth Circuit is called the third-party doctrine. Although an older form of the third-party doctrine was articulated in Ex Parte Jackson (1878), which distinguished between the addressing information for postal mail and its contents, the contemporary third-party doctrine traces its roots to the “reasonable expectation of privacy” test articulated in Katz v. United States(1967). Its modern form is most closely associated with two of Katz’s progeny, United States v. Miller (1976) and Smith v. Maryland (1979). In those cases, the Supreme Court applied Katz and concluded that the information at issue (bank records and information collected by a pen register device) was not entitled to Fourth Amendment protection. -- Featuring: Jim Harper, Vice President, Competitive Enterprise Institute and Orin S. Kerr, Fred C. Stevenson Research Professor of Law, The George Washington University Law School.