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Polychlorinated biphenyls are a family of 209 compounds made up of attached benzene rings with varying numbers and locations of chlorine atoms. PCBs are characterized by their low flammability, low electrical conductivity, high resistance to thermal breakdown and to other chemical agents, and high degree of chemical stability. These qualities make them effective coolants, lubricants, and insulators. The physical attributes that make PCBs useful in industry also make them a serious health threat to workers, wildlife, and communities. PCBs’ environmental persistence and long-range transport properties facilitate their movement to all environmental compartments. PCBs continue to enter the groundwater, soils, and atmosphere from multiple sources including old industrial equipment, recycled oil and materials, chemical manufacturing, landfills, and incineration of industrial and municipal waste. Even so called “ closed” industrial systems can release large amounts of PCBs. Until recently it was believed that there were no natural sources of PCBs. However, PCBs not associated with anthropogenic activities were identified in ash from volcanic eruptions, and subunits of PCBs have also been identified as components of glycoproteins. Understanding how the chemical behaves in the environment provides information on possible effects on human health and allows for the risk to humans and the environment to be assessed. In order to assess the behavior of a chemical, the intrinsic properties and emission patterns of the chemical must be known. Due to the persistence of POPs, and their tendency to bioaccumulate and biomagnify, almost undetectable concentrations in the abiotic environment have the potential under certain conditions to result in significant exposure levels for organisms at higher trophic levels, e.g. humans (with biomagnification factors greater than 107 in some cases). PCBs concentrate in the food web and bioaccumulate in the fatty tissue of wildlife and people. Virtually everyone has PCBs in their bodies. In order to understand distribution and transformations of PCBs in the environment, we need, besides their basic physical characteristics, additional information on their environmental behaviour and reactions. Even a good knowledge of physical and chemical properties of these persistent industrial substances does not enable precise distribution of PCBs over the Earth since such a global experiment was performed only once, and because it is complex, long-lasting, unplanned, subject to changes, and practically impossible to reconstruct. Thus, for complete understanding of distribution phenomena it is necessary to combine results of monitoring and field studies with the mathematical models, which are substitutes for the controlled experiment that cannot be performed in the natural systems.