Dioxins and related compounds are chlorinated aromatic hydrocarbons that are persistent in both environmental and biological samples. Many members of this class of compounds produce a similar spectrum of toxicity which is mediated by interaction with the Ah receptor. The toxic effects of these chemicals can best be described by their actions as growth dysregulators. Dioxins disrupt normal homeostatic processes that tightly regulate cellular growth and differentiation. Disruption in these processes produce a variety of toxicities and pathologies. The available data indicate that humans are sensitive to the toxic effects of these chemicals. Clearer definition of human responses and the body burdens associated with such effects requires more research. Comprehensive risk assessments of dioxins should include all Ah receptor ligands such as the halogenated dibenzofurans and biphenyls.