Commercial radioactive sources provide benefits through numerous applications in medicine, industry, and research. But some of these materials, if not secured, may end up in radiological dispersal devices (RDDs), also known as dirty bombs. Since al Qaeda’s expressed interest in acquiring the means to obtain radiological capability, there is a renewed concern about the security of commercial radioactive sources. This study focused on the security of commercial radioactive sources because they represent a significant category of radioactive materials that are used widely and have not previously been considered high security risks. The study found that only a small fraction of the millions of commercial radioactive sources used globally posed high security risks because of their portability, dispersibility, and higher levels of radioactivity. The more dangerous commercial sources were those containing relatively large amounts of radioactivity of seven reactor-produced radioisotopes. Some of these isotopes would only pose internal health hazards by means of ingestion or inhalation, while the others would present both internal and external health hazards because the emitted ionizing radiation could penetrate the dead outer layer of human skin. Radiological terrorists would seek very highly radioactive sources that pose external and internal health hazards to maximize harm to the targeted population. Terrorists might not live long enough to deliver an RDD because they might receive lethal acute doses of ionizing radiation from these sources in the absence of adequate shielding surrounding the radioactive material. Adding heavy protective shielding could substantially increase the difficulty in transporting an RDD. This could dissuade terrorists from employing these types of sources. Sources that present an internal health hazard and contain very high amounts of radioactivity could be handled safely without heavy shielding as long as precautions are taken to minimize internal exposure. Properly regulating and securing the smaller subset of resources could contribute significantly to reducing the dangers posed by commercial radioactive sources. 2 appendices