A considerable body of research over the past fifteen years establishes that in laboratory animals the Ah (aromatic hydrocarbon) receptor (AhR) mediates most, if not all, toxic effects of halogenated aromatic hydrocarbons such as polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans, and polyhalogenated biphenyls. More recently the AhR has been shown to also exist in a wide variety of human tissues and human cell lines. In general the AhR in humans appears to function very much like the AhR in rodents. However, the affinity with which toxic HAHs such as 2,3,7,8-tetrachlorodibenzo-p-dioxin bind to the AhR from human sources generally is lower than the affinity with which these HAHs bind to the Ah receptors from rodent tissues. This lower affinity may explain, in part, why the human species seems less sensitive than many laboratory animals to the effects of HAHs. The AhR enhances transcription of genes encoding cytochrome P450 enzymes in the CYP1A subfamily, but most of the toxic effects of HAHs do not seem to require P450 induction per se. Recent molecular approaches to the mechanism of HAH toxicity indicate that the AhR also may mediate expression of several other genes, including genes that regulate cell growth and differentiation. Despite the expanding repertoire of cellular responses known to be altered by HAHs (potentially through the AhR) it is not yet clear which AhR-mediated actions are the key events in HAH toxicity. Within the past year two subunits of the AhR have been cloned; this cloning, along with other molecular investigations, should greatly expand our opportunity to understand the specific mechanisms and pathways by which HAHs cause toxicity.