The cytochromes P450 (CYPs) are key enzymes in cancer formation and cancer treatment. They mediate the metabolic activation of numerous precarcinogens and participate in the inactivation and activation of anticancer drugs. Since all CYPs that metabolize xenobiotics are polymorphic, much emphasis has been put on the investigation of a relationship between the distribution of specific variant CYP alleles and risk for different types of cancer, but a consistent view does not yet exist. This is to a great extent explained by the fact that the CYPs involved in activation of precarcinogens are in general not functionally polymorphic. This is in contrast to CYPs that are active in drug biotransformation where large interindividual differences in the capacity to metabolize therapeutic drugs are seen as a consequence of polymorphic alleles with altered function. This includes also some anticancer drugs like tamoxifen and cyclophosphamide metabolized by CYP2D6, CYP2C19 and CYP2B6. Some P450 forms are also selectively expressed in tumours, and this could provide a mechanism for drug resistance, but also future therapies using these enzymes as drug targets can be envisioned. This review gives an up-to-date description of our current knowledge in these areas.