This report, which will be updated as necessary, presents a means of assessing the relative threat from terrorist-use of individual chemical, biological, and toxin agents. It focuses on small-scale, targeted chemical and biological attacks, rather than mass-casualty attacks. The framework considers the elements of access, public health impact, medical treatment, prophylaxis, and dissemination. Other factors that may affect potential use by terrorists include the range of lethality, covert employment of an agent, and the availability of dual-use technologies. The results of this framework may be useful in addressing the threat these agents pose, for example by indicating priorities for countermeasure funding. Other uses include weighing the potential effectiveness of policy options, assessing threat reduction approaches to specific agents, and serving as a resource for developing other specialized frameworks. Defense against chemical and biological agents is high on the list of the nation's priorities. No clear consensus exists with respect to which agents pose the greatest threat. Previous analyses of the chemical and biological threat have largely revolved around historical and comparative treatments or been based in a military framework. Examination of the chemical and biological threat to civilians is more complicated. Agents whose characteristics make them poor military weapons may still be powerful if deployed as weapons of terror. Chemical and biological weapons used in the past have not always been chosen for the highest potential fatalities, but rather for other reasons. Some chemical and biological agents are closely regulated, both domestically and internationally. Expansion or further refinement of policies controlling these agents may lower the threat posed by terrorist use of them. Domestic policy options to reduce the threat posed by these agents include methods to prevent their use, consequence management after their use, and methods for protecting the public from them. Specific policies to implement these goals include improving the general public health system, increasing prophylaxis research, development of new medical countermeasures treatments, increasing intelligence gathering, and increasing regulation of dual-use technology. International policy options include development of new biosecurity agreements and increasing participation in current non-proliferation organizations. It is impossible to eliminate the risk of chemical or biological terrorism. Important issues facing policymakers include balancing the need for increased security with the potential economic costs associated with increased regulation and redirected federal resources, determining the relative ratio between general and specific countermeasures against chemical and biological terrorism, and assessing the success of federal efforts at reducing chemical and biological terrorism vulnerability.