Evidence supporting the conclusion that 7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrenes are ultimate mutagenic and carcinogenic forms of benzo[a]pyrene (BP) is summarized. Qauntum mechanical calculations that predict reactivity of diol epoxides derived from BP and other polycyclic aromatic hydrocarbons are described. The calculations predict that diol epoxides in which the oxirane ring forms part of a "bay region" of a tetrahydrobenzo ring should be the most reactive for a given aromatic hydrocarbon. Experiments with dihydrodiols and diol epoxides from benzo[a]anthracene (BA) are described. The ability to metabolically activate BA 3,4-dihydrodiol to species much more mutagenic that those obtained from other BA dihydrodiols ant the much greater mutagenicity of the diastereoisomeric 3,4-diol 1,2-epoxides of 1,2,3,4-tetrahydro BA relative to other diol epoxides of BA are in accord with predictions of the quantum mechanical calculations.