Early knowledge of the potential human metabolism and pharmacokmetics of new drug candidates is of great importance at the several stages of drug development. The design, interpretation, and relevance to humans of animal toxicology studies are critically dependent on interspecies comparisons of routes of metabolism and pharmacoltoxico-kinetics. In early Phase I studies, prediction of human drug clearance and clearance pathways aids in dose-ranging, choice of sampling schedules, and analysis for potential metabolites. In Phase II development, drug interaction studies can be targeted to those likely to be relevant and clinically important with major savings in development time and resources. Knowledge that the metabolism of a new drug is likely to be subject to genetic polymorphism (such as the poor debrisoquine or poor mephenytoin metabolizer phenotypes) also can guide decisions at several stages in development. At a very early stage, the drug may be dropped or an analogue not subject to the polymorphism chosen as a better development candidate. In early Phase I studies, volunteer subjects can be phenotyped or genotyped, so that affected individuals can be excluded to reduce variability, or observed more closely for adverse effects. At the later stages in clinical development, knowledge of pharmacogenetic influences can help explain population variability in pharmacokinetics, therapeutic response, and occurrence of adverse effects. This review describes the recent advances in the development of in vitro methods to predict human drug metabolism and pharmacokinetics before in vivo studies, animal, or human are discussed in this chapter.