The heightened public awareness that environmental contaminants can act on the nervous system and have effects that appear in behavior presents a major challenge to behavioral scientists in all areas. They are being asked to conduct both hazard assessment (are there neurobehavioral effects of some chemical at any dose?) and risk assessment (what is the risk to a population at a specific level of exposure?) for behavioral effects that are not always well understood. The heavy metal lead provides an excellent example. Concern over lead poisoning lies in claims that it lowers scores on IQ tests, retards academic performance, and results in disruptive and even criminal behavior. The problems posed are daunting: How easy is it to detect a 5-point drop in scores on IQ tests? Can scientists predict these effects from experimental models using laboratory animals? Can they identify behavioral and neural mechanisms that account for these effects? What economic costs are incurred if the average IQ drops a few points, and how does this cost compare with that of reducing lead in the environment?These concerns are reflected in policymaking. Federal legislation and regulation of toxic substances specifically include behavior, including schedule-controlled operant behavior, as a regulatory endpoint (Tilson, 1990). The inclusion of nervous system damage in general, and its behavioral manifestations in particular, represents a sea change in public concern over unintended exposure to chemicals. Where cancer has been (and still is) a significant concern, the recognition that adverse behavioral effects follow certain types of chemical exposure is increasing. These effects carry over into a large number of domains. Environmental contaminants, even at very low exposure levels, contribute to disorders across the life span, including developmental disabilities and the hastening of age-related impairments. Behavioral scientists' understanding of how this happens not only has public policy implications but can also inform them about the behavior and neurobiology surrounding these disorders.