The field of cytochrome P450 (P450) research has developed considerably over the past 20 years, and many important papers on the roles of P450s in chemical toxicology have appeared in Chemical Research in Toxicology. Today, our basic understanding of many of the human P450s is relatively well-established, in terms of the details of the individual genes, sequences, and basic catalytic mechanisms. Crystal structures of several of the major human P450s are now in hand. The animal P450s are still important in the context of metabolism and safety testing. Many well-defined examples exist for roles of P450s in decreasing the adverse effects of drugs through biotransformation, and an equally interesting field of investigation is the bioactivation of chemicals, including drugs. Unresolved problems include the characterization of the minor "orphan" P450s, ligand cooperativity and kinetic complexity of several P450s, the prediction of metabolism, the overall contribution of bioactivation to drug idiosyncratic problems, the extrapolation of animal test results to humans in drug development, and the contribution of genetic variation in human P450s to cancer incidence.