Toxins are harmful biological agents produced by bacteria, plants and animals. They differ from chemical agents in that they are naturally occurring, are nonvolatile and, except for mycotoxins, are not dermally active. Toxins can be much more toxic than chemical agents. They are also not persistent in the environment and are not transmittable from person to person. Toxins that are stable in the environment, highly toxic and easy to produce are candidates for use as biological weapons. Some toxins, such as mycotoxins and ricin, require much larger quantities than toxins such as botulinum to be effective in an open-air aerosol exposure; however, they can be effectively used in an indoor release. In this chapter, we will discuss the four toxins that are considered most likely to be used as biological weapons either by state entities or terrorist organizations. The list includes one highly lethal toxin, botulinum, two fairly lethal toxins, ricin and mycotoxins, and one that is considered primarily an incapacitating agent, staphylococcal enterotoxin B. Botulinum is a Center for Disease Control and Prevention (CDC) Category A agent, while the other toxins are CDC Category B agents.