Polychlorinated biphenyls represent a class of chlorinated compounds, the general structure for which is given in Figure 6.1. Each of the numbered positions may or may not be chlorinated, resulting in 209 different congeners. For industrial purposes, PCBs were manufactured as complex mixtures containing from 60 to 90 congeners by the catalytic chlorination of biphenyl (Shulz et al., 1989). Depending on the amount of chlorine added, these mixtures were mobile oils, viscous liquids, or sticky resins, but all were nonflammable, thermally stabile, chemically inert, and excellent electrical insulators. Because of these properties, they were widely used as dielectric fluids in electrical capacitors and transformers and as plasticizers. Smaller but still significant amounts were used as lubricants, hydraulic fluids, heat transfer fluids, cutting oils, extenders in waxes, pesticides, and inks, and in carbonless copy paper (Hutzinger et al., 1974).