Halogenated aromatic xenobiotics such as the chlorinated and brominated biphenyls, naphthalenes, dibenzodioxins, and dibenzofurans are widespread environmental contaminants. The number, position, and nature of the halogen atoms as well as the structure of the aromatic rings influence the disposition of these chemicals in living systems. Absorption is governed primarily by the physical properties of lipophilicity and solubility. Distribution through the blood occurs by nonspecific binding to plasma proteins and cellular components. Liver and adipose tissue are the major depots. Metabolism is a prerequisite for excretion. The highly substituted isomers tend to be resistant to metabolism. The route of excretion shifts from urine to feces with increasing size and number of halogen atoms. Although pharmacokinetic modeling has allowed some predictions to be made from one compound to another or across species, more information on metabolism is required in order to improve the ability to predict the disposition in humans of this class of toxic environmental pollutants.