The ubiquity of cell phones has transformed police investigations. Tracking a suspect’s movements by following her phone is now a common but largely unnoticed surveillance technique. It is useful, no doubt, precisely because it is so revealing; it also raises significant privacy concerns. In this Note, I consider what the procedural requirements for cell phone tracking should be by examining the relevant statutory and constitutional law. Ultimately, the best standard is probable cause; only an ordinary warrant can satisfy the text of the statutes and the mandates of the Constitution.