In a significant ruling in the fall of 2010, the Third Circuit Court of Appeals rejected the government’s claim that it could compel cell phone service providers to disclose customer records that indicate the cell towers with which a cell phone has communicated (cell phone location information or CSLI) without obtaining a warrant based on probable cause. In a break with past decisions, the court rejected application of a “third party rule,” under which cell phone users are seen to assume the risk that their providers will disclose location data without the protections of a warrant requirement. The court, however, remanded the case to the magistrate judge to determine whether a warrant is required, and explained that it is a question of fact whether acquisition of location data intrudes upon reasonable expectations of privacy. In this article, I argue that the question is instead one of law; courts should require a warrant for access to location data in all cases because such acquisition is a search under the Fourth Amendment. I address three factors that have impeded proper resolution of the constitutional question: 1) a lack of understanding of the power of cell phone location information to provide a detailed portrait of subjects’ private lives, 2) an unduly constrained application of the Knotts and Karo precedents concerning use of radio tracking beepers, and 3) the appeal of analytical short cuts such as an inappropriate extension of the Miller v. United States and Smith v. Maryland cases to cover location data. A full understanding of the power of the technology and a proper application of the constitutional precedents yields the conclusion that compelled acquisition of cell location data requires a warrant based on probable cause and may not be acquired on the lesser authority of a court order based on mere relevance.