Polychlorinated biphenyls (PCBs) are industrial compounds that have been detected as contaminants in almost every component ofthe global ecosystem including the air, water, sediments, riu, and wildlife and human adipose tissue, milk, and serum. PCBs in commercial products and environmental extracts are complex mixtures of isomers and congeners that can now be analyzed on a congener-specific basis using high-resolution gas chromatographic analysis. PCBs are metabolized prarily via mixed-function adxdaes intoa broadspectrum of m-tIbolites. The resuts indicate that metabolk activation is not required for PCB toxicity, and the parent hydrocarbons are responsible for most of the biochemical and toxic responses elicited by these compounds. Some of these responses include developmental and reproductive toxicity, dermal toxicity, endocrine effects, hepatotoxicity, carcinogenesis, and the induction of diverse phase I and phase I drugmetabolizing enzymes. Many of the effects observed for the commercial PCBs are similar to those reported for 2,3,7,-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds. Strctur-function relaionships for PCB congeners have identified two major stnctural classes ofPCBs that elicit "TCDD-lik"' responses, namely, the coplanar PCBs (e.g., 3,3',4A,'-tetraCB, 3,3' 4,4 ,5-pentaCB and 3,3',4,4,,5'-hexaCB) and their mono-ortho coplanar derivatives. These compounds competitively bind to the TCDD or aryl hydrocarbon (Ah) receptor and exhibit Ah receptor agonist activity. In addition, other structural dasses of PCBs elicit biochemical and toxic responses that are not mediated through the Ah receptor. The shor-term effects of PCBs on occupationally exposed humans appear to be reversible, and no consistent changes in overall mortality and cancer mortality have been reported. Recent studies have demonstrated that some developmental deficits in infants and children correlated with in utero exposure to PCBs; however, the etiologic agent(s) or structural clas of PCBs responsible for these effects have not been dlinated. In contrast, based on a toxic equivalency factor approach, the reproductive and developmental problems in certain wildlife populations appear to be related to the TCDD-like PCB congeners.