Polycyclic aromatic hydrocarbons (PAHs) are produced during the incomplete combustion of organic material. They can also be produced through natural, non-combustion processes, and may be present in uncombusted petroleum. Uncombusted petroleum can be a direct source to the waters of the Great Lakes, but combustion sources discharge PAHs into the coastal atmosphere. Atmospheric deposition of combustion related PAHs seems to be the dominate source to the Great Lakes, except in nearshore areas where point sources can be significant. Once airborne, PAHs partition in the atmosphere between the gas and particle phases and can undergo long-range transport. During transport, PAHs can be degraded or modified by photochemical reactions. Both the original PAH species and their degradation products can be washed out of the atmosphere by wet and dry deposition, air-water exchange and air-terrestrial exchange. Once in an aquatic system, PAHs partition between the dissolved and particle phases. In general, PAHs are particle reactive and settle out in sediments. PAH contamination of Great Lakes sediments are higher in the nearshore regions where ports, harbors, and urban/industrial areas are the densest. In the open lake area, sediment concentrations are rather uniform, with Lake Superior having slightly less PAHs in its surficial sediments. That portion of the PAHs that does not partition to particles can bioaccumulate in the lipid reserves of organisms. PAHs accumulated in an organism may be metabolized to more toxic by-products or exert toxicity in its original form. When combined with ultraviolet radiation this toxicity is greatly enhanced. In coastal areas where concentrations can be quite high, PAHs can be toxic to all forms of aquatic life during at least part of their life cycle. PAHs are expected to remain an ecological threat to the Great Lakes well into the future. The threat may even increase with the increasing combustion needed for the increasing population centers and greater transportation needs. Of particular concern is the short-term increase in PAH concentrations that can result from the dredging of ports and harbors where highly contaminated sediments have been buried.