One can conceive of at least three potentially catastrophic events involving the energy of the atom: a nuclear accident in which massive quantities of radiation inadvertently are released into the environment including inadvertent nuclear missile launches; nuclear war among nation-states; and nuclear violence inflicted by non-state actors. This chapter focuses on the last of these threats – the dangers posed by nuclear terrorism, a phenomenon that lies at the nexus between what are widely considered to be two of the primary security threats of the modern era. Non-state actors have essentially four mechanisms by which they can exploit civilian and military nuclear assets intentionally to serve their terrorist1 goals: • the dispersal of radioactive material by conventional explosives or other means; • attacks against or sabotage of nuclear facilities, in particular nuclear power plants and fuel storage sites, causing the release of radioactivity; • the theft, purchase, or receipt of fissile material leading to the fabrication and detonation of a crude nuclear explosive, usually referred to as an improvised nuclear device (IND); and • the theft, purchase, or receipt and detonation of an intact nuclear weapon. All of these nuclear threats are real; all merit the attention of the international community; and all require the expenditure of significant resources to reduce their likelihood and potential impact. The threats, however, are different and vary widely in their probability of occurrence, in consequences for human and financial loss, and in the ease with which intervention might reduce destructive outcomes (for a detailed analysis, see Ferguson and Potter, 2005). Nuclear terrorism experts generally agree that the nuclear terror scenarios withthehighestconsequences–thoseinvolvingnuclearexplosives–aretheleast likely to occur because they are the most difficult to accomplish. Conversely, the scenarios with the least damaging consequences – those involving the release of radioactivity but no nuclear explosion – are the most likely to occur because they are the easiest to carry out. Constructing and detonating an IND, for example, is far more challenging than building and setting off a radiological dispersal device (RDD), because the former weapon is far more complex technologically and because the necessary materials are far more difficult to obtain.