Over the last 30 yr, in vitro assessment of interactions between drugs and drug-metabolizing enzymes has developed from a scientific curiosity to a topic of major clinical and commercial importance. For clinicians, knowledge of this area assists in the ability to predict the likelihood of a drug-drug interaction, or may help explain certain cases in which there is unexplained toxicity or lack of efficacy. For the pharmaceutical industry, selection of appropriate compounds for further preclinical and clinical development is a critical issue. This chapter will introduce some basic concepts about the most important drug-metabolizing enzyme system in humans, the cytochrome P-450 (CYP-450) family of enzymes, with the emphasis on in vitro methods of assessment, and how these correspond with clinical studies.