Patricia McDonough was the victim of a robbery. She gave the police a description of the robber and of a 1975 Monte Carlo automobile that she had witnessed during the robbery. McDonough later received threatening phone calls from a man who claimed to be the robber. On one occasion, the man asked McDonough to step out on her front porch. McDonough did so and witnessed the 1975 Monte Carlo that she had earlier described to police moving slowly past her home. Police saw a man who met McDonough's description driving the same car in McDonough’s neighborhood. The police traced the license plate number and learned that the car was registered to Michael Lee Smith (defendant). Without obtaining a warrant, the police requested the telephone company to install a pen register at its central offices to record the numbers dialed from Smith’s home. The pen register showed that Smith later placed another call to McDonough. The State of Maryland (plaintiff) charged Smith with robbery. Prior to trial, Smith attempted to suppress the pen register because the police did not obtain a warrant. A jury convicted Smith, and the court of appeals affirmed. Smith appealed, arguing that the police's use of the pen register constituted an unlawful search under the Fourth Amendment and that he had a reasonable expectation of privacy in his home. The United States Supreme Court granted certiorari.