Since September 11, 2001, most expert commentary on radiological weapons has focused on nonstate actors, to the neglect of state-level programs. In fact, numerous countries in the past have expressed interest in radiological weapons; a number have actively pursued them; and three tested them on multiple occasions before ultimately deciding not to deploy the weapons. Why is so little known about these false starts, especially outside the United States? Are such weapons more difficult to manufacture than depicted by science-fiction authors and military pundits? Are radiological weapons a thing of the past, or do they remain an attractive option for some countries? A comparative analysis of the previously underexplored cases of radiological weapons programs in the United States and the Soviet Union illuminates the drivers and limitations of weapons innovation in one specific nuclear sector. An examination of the rise and demise of radiological weapons programs in both countries also points to circumstances in the future that might prompt renewed interest on the part of some states in radiological weapons and proposes steps that might be undertaken to reduce the possibility of their production, deployment, and use.