Does the diffusion of nuclear programs encourage nuclear and radiological (NR) terrorism? It is argued that a country's vulnerability to NR terrorism grows as the size of its nuclear program increases. The presence of expansive nuclear infrastructure facilitates terrorists efforts to steal or acquire nuclear and radiological materials, increases the targets open to attack, and amplifies the potential economic and social-psychological consequences of related terrorist attacks. It is also argued that countries can reduce the likelihood of NR terrorism by improving nuclear security and reducing corruption. An empirical analysis of NR terrorist incidents during 19922006 supports for these arguments. The findings offer empirically grounded insights into the future risks of nuclear terrorism, and possible solutions are offered.