Organohalogen compounds have been introduced in Chapter 6 (solvents) and Chapter 7 (pesticides). Polychlorinated aromatic compounds (PCAs), especially polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins (PCDDs) have received enormous attention by scientists, regulators, the press, and the public resulting in an inconceivable volume of printed information in two decades. This is more remarkable because during the first four decades of the five-decade commercial production of PCBs they were dismissed as being biologically inactive but extremely useful commercially. PCBs, PCDDs, and the closely related polychlorinated dibenzofurans (PCDFs) (Figure 8.1) have been examined from nearly every aspect of toxicology and ecotoxicology; yet there is no clear consensus on the past, present, or future impacts of these ubiquitous global contaminants.