Cytochromes P450 catalyse extremely diverse and often complex regiospecific and/or stereospecific reactions in the biosynthesis or catabolism of plant bioactive molecules. Engineered P450 expression is needed for low-cost production of antineoplastic drugs such as taxol or indole alkaloids and offers the possibility to increase the content of nutraceuticals such as phytoestrogens and antioxidants in plants. Natural products may serve important functions in plant defence and metabolic engineering of P450s is a prime target to improve plant defence against insects and pathogens. Herbicides, pollutants and other xenobiotics are metabolised by some plant P450 enzymes. These P450s are tools to modify herbicide tolerance, as selectable markers and for bioremediation.