This book focuses on the vulnerability of all open societies to loss of life from the covert or terrorist delivery of nuclear, biological, or chemical weapons. The passing of the Soviet threat has changed the nature of the mass-destruction threat to U.S. cities, making it necessary to update the strategic assumptions of defense policy. Nuclear, biological, and chemical (NBC) weapons share three characteristics: immense lethality, portability, and accessibility. A covert NBC attack could be carried out in peacetime or during war, and could target civilians, military forces, or infrastructure. The defining element of a covert attack is that the weapon is delivered against its target in a manner that cannot readily be distinguished from normal background traffic and activity. A covert NBC attack against the United States is a quintessential low-probability, high-consequence event. However, it is growing greater with time for three key reasons. These weapons are becoming more accessible to a wider range of groups. The predominance of military forces and the virtual invulnerability of the homeland to direct military attack leave international adversaries with few options other than unconventional threats. The nature of nonstate violence is changing in a way that strongly suggests risking NBC risks. The consequences of covert NBC attacks and terrorism are massive casualties, contamination, panic, degraded response capabilities, economic damage, loss of strategic position, and social-psychological damage and political change. Recommendations to reduce the vulnerability of the United States included establishing an NBC response center; reinvigorating the intelligence community, improving the detection of small-scale NBC weapons programs; and improving capability for post-attack attribution.