The intentional use of biological or chemical agents to cause disease or destroy food and water supplies for political or economic reasons dates to antiquity. The turn of the twentieth century heralded the development of lethal biological and chemical weapons capable of mass destruction. By the final two decades of the twentieth century, individuals and small groups learned how to obtain and use these weapons effectively. The events of the past few decades, including the World Trade Center and anthrax attacks in 2001, have shown that small groups of individuals, as well as nations, have the resources to coordinate attacks using chemical, biological and radiological agents. Given the long history of biological and chemical warfare, and given that that many of these agents are relatively easy to obtain and use, future attacks are possible. If they occur, primary care clinicians will have key roles to play in protecting their patients and the public. The illnesses that biological, chemical and radiological weapons cause can be difficult to distinguish from naturally occurring illness. Clearly, clinicians will need a basic understanding of diseases caused by these agents, including their associated epidemiology, and an understanding on how to work with public health officials to protect their patients and the public.