The resemblance of the collapse of the World Trade Center to the mushroom cloud of a nuclear explosion buttressed the burgeoning perception1 that WMD could become a weapon of choice for terrorists. This new concern contributed to the creation of the U.S. Department of Homeland Security (DHS). DHS, along with other federal agencies, has spent billions of dollars to prevent and respond to the use of nuclear weapons or radioactive materials by terrorists, and nuclear and radiological incidents figure prominently in the National Response Plan (NRP). Do these efforts represent an adequate response to the nature and magnitude of nuclear and radioactive threats? Is a comprehensive strategy in place, or are there gaps that must be addressed? Our answer to these questions is that, although preparations for responding to nuclear and radiological emergencies are well underway, current efforts at preventing these incidents have significant deficiencies. With regard to radiological attacks, which in general represent high-probability, low-consequence events, this situation may be tenable, though action is still required in some areas. However, further action to prevent low-probability, high-consequence nuclear events is urgently needed.