In March 1995, Japanese terrorists released nerve gas on the Tokyo subway, causing eleven deaths and more than 5,000 injuries. Although terrorists have sought to acquire chemical/biological (C/B) agents in the past, and a few have employed them on a small scale, the Tokyo attack was the first large-scale terrorist use of a lethal chemical agent against unarmed civilians, weakening a long-standing psychological taboo. This tragic incident has therefore drawn worldwide attention to the emerging threat of chemical/biological terrorism. Despite significant technical hurdles associated with the production and delivery of C/B agents, such weapons are within the reach of terrorist groups that possess the necessary scientific know-how and financial resources. This article proposes a C/B counterterrorism strategy based on preemption and civil defense, and recommends several short-term and longer-term policy options for mitigating this emerging threat.