Cytochrome P450 enzymes are haem-thiolate containing proteins which function as monooxygenases across almost all living organisms. The cytochrome P450 enzymes catalyse a multitude of oxidation reactions and, in humans have an important role in xenobiotic and drug metabolism. P450s perform hydroxylation, epoxidation, N- and O- dealkylation, and sulfoxidation of organic molecules (among many other reactions). They do this by using the haem-thiolate cofactor to activate dioxygen. This requires two distinct reduction steps. It would be appealing to exploit these enzymes as environmentally benign catalysts in the synthesis of fine chemicals. Their widespread use in industrial synthesis is hampered by the high cost of the cofactor NAD(P)H required for the reducing equivalents. Here I describe how P450s can be engineered to use cheap hydrogen peroxide (H2O2) to help overcome this problem. P450Bm3 (CYP102A1) from the bacterium Bacillus megaterium is an unusual P450 which has its electron transfer domain fused to the haem domain[1]. It hydroxylates the ω-1 ̴ ω-3 carbon atoms of selected fatty acids. Here I have used an engineered form of the haem domain, which functions as a peroxygenase, to oxidise fatty acids and aromatic compounds such as tetralone, propiophenone, 1-methoxynaphthalene, and 1-naphthol. Several of these reactions display high levels of regioselectivity. Therefore, this peroxygenase variant has the potential to act as a biocatalyst for the oxidation of a broad range of substrates. The optimization of the reaction conditions for this P450 peroxygenase and related enzymes will also be discussed.