One area of public health emergency preparedness that has not been examined in depth is preparedness for incidents involving the release of chemical or radiological substances. Past experience — with chemical and nuclear plant accidents, train collisions, product tampering, and chemical terrorism — shows that such incidents can have serious public health consequences. This report focuses on the roles of the public health service in emergency preparedness and its response to chemical and radiological incidents. The authors develop a functional framework for public health roles in such incidents by aligning the capabilities and roles of the public health service with the emergency preparedness and response activities that would be required for those incidents. They use this framework to examine representative state and local public health department practices and to identify areas where further practice development may be warranted.