The FBI's security function in special events is defined in numerous statutes and presidential directives. This authority, combined with the FBI's responsibilities in combating terrorism, provide the basis for FBI involvement in security for domestic special events. In the international environment, the FBI is responsible for postcrisis terrorism investigations. This authority extends to prevention efforts that could foil a plot that involves a foreign special event, such as the Athens Summer Olympics. The FBI's decision to become involved in security for a domestic or foreign special event is governed by the following criteria: size, threat, significance, duration, location, attendance, media coverage, dignitaries, and viewing audience. These criteria reflect the likelihood that terrorists will be motivated to plan and execute an attack at the event. In the case of the Athens Summer Olympics, the U.S. Government's security efforts extended beyond the traditional concerns about physical security at venue sites, transportation security, and credentials. Security concerns also included the collection, analysis, and dissemination of intelligence; and proactive investigations into credible and specific information relevant to the event. This article describes the training, preparatory exercises, and operational structure for U.S. Government agencies involved in security for the Athens Summer Olympics. The components of the "after-action report" following the event are also discussed.