GPS devices enable law enforcement agencies to track the movement of vehicles of interest while creating a permanent and accurate record of where the vehicle was and the time it was there. In order to use this technology, officers must have lawful access to the target vehicle in order to install certain instruments. The simplest form of installation consists of a GPS receiver, antenna, power supply, and logging device. Depending on the equipment, officers can remotely obtain data electronically or by physically retrieving the logging device from the vehicle. On December 1, 2006, Rule 41 of the Federal Rules of Criminal Procedure was modified to include procedures for Federal agents to obtain, process, and return warrants to install and use tracking devices. In general, the rule allows for a magistrate judge to issue a warrant that authorizes the installation of a tracking device. It requires that a return be made in order to inform the issuing magistrate judge about the details of the installation and the notice to be provided to the target of the order. This article describes a case in which GPS tracking was used to monitor the movement of four suspects believed to be involved in the harassment and threatening of witnesses in the trial of a defendant charged with firearms and drug-trafficking offenses. The case describes the features and installation of the GPS technology used in each of the tracked vehicles. Relevant legal issues and case outcome are also discussed. 20 notes