Chemical and biological experts and intelligence agency officials believe that the ease or difficulty for terrorists to cause mass casualties with an improvised chemical or biological weapon or device depends on the chemical or biological agent selected. Experts from the scientific, intelligence, and law enforcement communities told us that terrorists do not need sophisticated knowledge or dissemination methods to use toxic industrial chemicals such as chlorine. In contrast, terrorists would need a relatively high degree of sophistication to successfully cause mass casualties with some other chemical and most biological agents. Specialized knowledge would be needed to acquire the right biological agent or precursor chemicals,3 process the chemical or biological agent, improvise a weapon or device, and effectively disseminate the agent to cause mass casualties. Throughout the different stages of the process, terrorists would run the risk of hurting themselves and of being detected and would have to overcome technical and operational challenges. Some virulent biological agents and precursor chemicals are difficult to obtain, and others are difficult to process or produce, especially in the quantities needed to cause mass casualties. In addition, effective dissemination of chemical and biological agents can be disrupted by environmental and meteorological factors. Terrorists with less sophistication could make a chemical or biological weapon and disseminate agents, but these would be less likely to cause mass casualties.