Polynuclear aromatic hydrocarbons (PAHs) can enter the environment in several ways, primarily through the incomplete combustion of carbonaceous materials or through processes that convert coal into synthetic fuels. Other sources of PAHs include the manufacture of carbon black, creosote, soot, vehicular emissions (especially diesel), residual oil, and wood smoke. The principal path of human exposure is through inhalation. Some PAHs are of concern because they can become mutagenic or carcinogenic after being metabolized. However, certain substituted PAHs seem to exhibit direct mutagenic or carcinogenic activity. Some of the most vigorous research activity related to PAhs has concentrated on refining analytical techniques and on development of a better understanding of PAH-induced mutagenesis and carcinogenesis at a molecular level. Selected studies are described in the review.