In 1992, the Secret Service, in partnership with the National Institute of Justice and with assistance from the Federal Bureau of Prisons, began the Exceptional Case Study Project (ECSP), a 5-year study to examine the thinking and behavior of individuals who have attacked or approached to attack prominent public officials or figures in the United States since 1949. ECSP findings reveal general threat assessment information relating to attacks on public officials and figures, while suggesting that broader application of threat assessment protocols by Federal, State and local law enforcement officials could help anticipate and prevent other crimes, such as stalking and workplace violence. Drawing from project findings, this guide describes an approach to threat assessment and the protective intelligence investigative process that can be of assistance to Federal, State, and local law enforcement and security professional with protective intelligence responsibilities. Though not intended to serve as an operations manual, the guide presents information and ideas about developing and implementing protective intelligence programs and activities. Information about the thinking and behavior of persons who have attacked or come close to attacking public officials and figures can help refine law enforcement operations related to preventing and investigating violence and threats of violence.