This report considers the potential for terrorists to use biological weapons. It discusses the implications of recent trends in terrorism for the future use of biological agents and the reasons terrorists might be motivated to use them. It then identifies several constraints that inhibit terrorists from venturing into this new type of conflict and the factors that could break down these constraints. Finally, it establishes some broad characteristics that could identify the types of terrorist groups that might be more likely than others to use biological weapons. The findings suggest that, since the technological, logistical, and financial barriers to the use of biological agents are not insurmountable, a key determinant in the potential use of such agents will be the willingness of terrorists to engage in this new type of violence. Therefore, efforts to improve intelligence regarding terrorist group strategies and capabilities will become increasingly critical in the future.