Polycyclic aromatic hydrocarbons are generated through inefficient combustion and, while initially released largely into the atmosphere, they are subsequently deposited in soil and water. They contaminate marine sources of food as well as vegetables and plants and can be generated during the smoking or broiling of fish and meat. The metabolic fate of hydrocarbons in mammalian systems has been extensively studied and, while many hydrocarbons are noncarcinogenic and efficiently detoxified, small fractions of some hydrocarbons are converted to electrophilic metabolites which are not effectively further metabolized and which are probably responsible for the carcinogenic properties of these hydrocarbons. These electrophilic metabolites are probably the bay-region dihydrodiol-epoxides, but at present the tumorigenic properties of the bay-region dihydrodiol-epoxides that have been tested are not as great as might be expected.