A considerable amount of information has been obtained about human enzymes involved in drug metabolism. Cytochrome P450 is presented as a paradigm in order to illustrate the experimental techniques now available. In vitro assays can be done with tissue slices, microsomes, or even short-term cell cultures. In addition, recombinant enzymes can be produced in a variety of different vector systems. Information about catalytic selectivity regarding new drugs can be obtained, as well as important information about potential drug-drug interactions and bioavailability. Such studies play a role in the drug development process as well as metabolism and safety assessment.