The possibility that criminals or political extremists might sabotage nuclear facilities, fabricate a nuclear explosive device, or disperse radioactive material has been the subject of numerous studies. What conclusions can we draw? (1) It can be done. (2) There are political extremists and criminal groups today that possess or could acquire the necessary resources. (3) The historical record provides no evidence that such groups have attempted to acquire nuclear material for illicit use; there have been low-level incidents involving nuclear facilities or material. (4) There is no inexorable progression from the currently identified spectrum of potential nuclear terrorists to actual nuclear terrorists, or from the nuclear incidents that have occurred to actions of greater consequence. (5) Whether any of the current potential nuclear terrorists will actually go nuclear remains an unanswerable question. (6) There may appear new kinds of adversaries more likely to take nuclear action. (7) The level and nature of the threat may change.