In most cases terrorists would need to overcome significant technical and operational challenges to successfully make and release chemical or biological agents of sufficient quality and quantity to kill or injure large numbers of people without substantial assistance from a state sponsor. With the exception of toxic industrial chemicals such as chlorine, most chemical and nearly all biological agents that could be used in terrorist attacks require specialized knowledge for their manufacture. Improvising effective delivery devices also requires specialized knowledge. Moreover, some of the necessary components of chemical agents and highly infective strains of biological agents are difficult to obtain. Finally, for a successful attack terrorists may need to overcome other obstacles, such as unfavorable environmental conditions and personal safety risks. A national-level assessment of the risk of chemical and biological terrorism, based on analyses of both the foreign- and domestic-origin threats, could help determine the requirements and priorities for combating terrorism and help target resources where most needed. Notes, figure, references