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Heterologously expressed human drug-metabolizing enzyme systems in a variety of hosts (mammalian cell lines, baculovirus/insect cells, yeast and Escherichia coli) have their distinct advantages for particular studies of biotransformation or mechanistic processes. In contrast to classical in vitro systems such as tissue slices, hepatocytes or subcellular fractions, expressed enzymes allow the study of single enzyme reactions in isolation. Furthermore, metabolic reactions of enzymes expressed only in minor amounts in human tissue can be assessed. Here we present an overview of how recombinant enzymes are being used in biotransformation studies and we will present several examples of applications of recombinant cytochrome P450 preparations during drug development. It is beyond the scope of this overview to describe all experimental procedures in detail since they are based on published techniques unless otherwise indicated. Examples of applications of recombinant cytochrome P450 preparations include the involvement of human metabolizing enzymes in a metabolic pathway and their metabolic products, mechanistic studies to determine specific drug-drug interactions at the metabolic level and detection of mechanism-based inactivation of drug-metabolizing enzymes. Heterologous expression systems offer a constant and reproducible source of human drug-metabolizing enzymes that are easily available by standard laboratory techniques. Considering the constraints on the availability and use of human tissue it is likely that these systems will be widely used in the future. But, because of the unequal distribution of individual drug-metabolizing enzymes in man, extrapolation needs to be done carefully.