There is great concern that terrorists could obtain nuclear or radiological weapons and detonate them in a large city. The authors analyse the technical requirements for and obstacles to obtaining such weapons. What difficulties would have to be surmounted? Could these problems be solved by a terrorist organization without direct support from a State possessing nuclear weapons? The authors conclude that nuclear weapons are most likely out of reach for terrorists. However, radiological weapons may well be used by terrorists in the future. The possible consequences of such an attack are discussed.