Studies of xenobiotic biotransformation by in vitro techniques are destined to play an increasingly important role in assessing the safety and efficacy of drugs and other new chemical entities. The first part of this article summarizes some of the in vitro techniques that have been developed to evaluate xenobiotics as inducers of liver microsomal cytochrome P450. The second part provides an overview of reaction phenotyping, an in vitro technology for determining which human P450 enzyme or enzymes are involved in the biotransformation of xenobiotics.