The aromatic hydrocarbon (AH) (dioxin) receptor was discovered almost 20 years ago and achieved notoriety as the front-line site of action of highly toxic environmental chemicals such as halogenated dioxins and polychlorinated biphenyls. Increasing evidence suggests that the AH receptor plays a key role in proliferation and differentiation of cells exposed to dioxins and, perhaps, to endogenous ligands. Recent cloning of the AH receptor and its indispensable partner, the AH-receptor-nuclear-translocator protein, has opened new opportunities to determine how the AH receptor functions, how it evolved and what its multiple roles might be in normal physiology as well as in toxicology. This review by Allan Okey, David Riddick and Patricia Harper aims to provide a brief history of AH receptor research and gives a timely summary of what is known and what is not known about the structure and function of this fascinating protein.