The nuclear/radiological terrorist attack probability seems a serious brainteaser in the contemporary security discourse, and transnational terrorist groups have been employing modern scientific know-how to accomplish their nefarious objectives. Since 1995, there have been several cases, which underscore the fact that the terrorist groups could have access to–and no scruples about using–weapon of mass destruction materials. Nuclear materials range from highly enriched uranium in nuclear power reactors and military stockpiles to plutonium in a nuclear reactor's spent fuels, to radiological sources in local hospitals and agriculture research centers. If nuclear materials or radioactive sources fall into the hands of terrorist groups, it engenders enormous harm to human life and property. As a consequence, to counter radiological/nuclear terrorist attacks effectively standards for securing weapons and materials should be so high that terrorists will not be able to exploit any compromises or gaps in the defenses. That's why all countries with nuclear material holdings have to apply the most up-to-date IAEA guidelines on nuclear security.