In light of the exceptional circumstances that arose from hosting the Olympic Games in Athens in 2004 and from recent terrorist events internationally, Greece attributes the highest priority to security issues. According to its statutory role, the Greek Atomic Energy Commission is responsible for emergency preparedness and response in case of nuclear and radiological events, and advises the Government on the measures and interventions necessary to protect the public. In this context, the Commission participated in the Nuclear, Radiological, Biological, and Chemical Threat National Emergency Plan, specially developed for the Olympic Games, and coordinated by the Olympic Games Security Division. The objective of this paper is to share the experience gained during the organization of the Olympic Games and to present the nuclear security program implemented prior to, during, and beyond the Games, in order to prevent, detect, assess, and respond to the threat of nuclear terrorism. This program adopted a multi-area coverage of nuclear security, including physical protection of nuclear and radiological facilities, prevention of smuggling of radioactive materials through borders, prevention of dispersion of these materials into the Olympic venues, enhancement of emergency preparedness and response to radiological events, upgrading of the technical infrastructure, establishment of new procedures for assessing the threat and responding to radiological incidents, and training personnel belonging to several organizations involved in the National Emergency Response Plan. Finally, the close cooperation of Greek Authorities with the International Atomic Energy Agency and the U.S. Department of Energy, under the coordination of the Greek Atomic Energy Commission, is also discussed.