Catalytically active cytochrome P450 enzymes have been successfully expressed in bacterial, yeast, and mammalian cells. A variety of expression vectors have been used, resulting in both transient and stable expression. The system of choice depends on the goals of a particular project. Factors such as expense, ease of use, and yields required should govern the decision whether to use bacterial, yeast, insect, or mammalian cDNA expression. High-level expression of mammalian P450s in bacteria usually requires modifications of the amino-terminal region of the enzyme. The Escherichia coli P450-OR fusion proteins may also come of age for use in fermentation-production processes for the chemical industry. Many cytochromes P450 have been expressed in yeast, with variable levels of expression. Baculovirus, albeit somewhat tedious in having to individualize expression conditions, can produce high levels of enzyme. The standard mammalian cell expression systems, both transient and stable, have been of tremendous value to drug metabolism and carcinogenesis research and will continue to play a role in these areas.