Diverse oxygenation patterns of natural products generated by secondary metabolic pathways in microorganisms and plants are largely achieved through the tailoring reactions catalysed by cytochrome P450 enzymes (P450s). P450s are a large family of oxidative hemoproteins found in all life forms from prokaryotes to humans. Understanding the reactivity and selectivity of these fascinating C-H bond-activating catalysts will advance their use in generating valuable pharmaceuticals and products for medicine, agriculture and industry. A major strength of this P450 group is its set of established enzyme-substrate relationships, the source of the most detailed knowledge on how P450 enzymes work. Engineering microbial-derived P450 enzymes to accommodate alternative substrates and add new functions continues to be an important near- and long-term practical goal driving the structural characterization of these molecules. Understanding the natural evolution of P450 structure-function should accelerate metabolic engineering and directed evolutionary approaches to enhance diversification of natural product structures and other biosynthetic applications.