The cytochromes P450 are members of a class of enzymes known oxygenases, as the incorporate an oxygen atom from atmospheric dioxygen into a substrate molecule. As mono-oxygenases, the P450s utilize two electrons from NAD(P)H to reductively cleave atmospheric dioxygen, reducing one atom to water and using the second to oxidize the substrate. Mechanistically, this involves a multistep mechanism to form reactive iron-oxygen adducts as shown in the Scheme 1. The cytochrome P450s represent one of the largest superfamilies of enzymes, with over 25,000 identified sequences from all branches of life. In general these enzymes operate in both catabolic and anabolic roles. In humans, the enzymes are involved in breakdown of xenobiotics such as ingested therapeutics and environmental compounds as well as in the synthesis of the steroid hormones and prostaglandins. Detailed understanding of P450 mechanisms is critical to biochemists, pharmacologists, toxicologists and chemists. Our major efforts are focused on isolating and characterizing the reactive intermediates of the P450 catalytic cycle, the modes of molecular recognition of enzyme for its substrate and the details of inter-protein electron and proton transfer. Recent focus is on the enzymes involved in human drug metabolism (e.g. CYP3A4 and CYP2C9) and steroid biosynthesis (CYP17 and CYP19). For the drug metabolizing P450s, we seek to understand the molecular mechanisms of drug-drug interaction and the role of protein dynamics in linking the allosteric and catalytic sites of the enzymes. In the case of steroid biosynthesis, we are identifying the specific catalytic intermediate involved in catalysis with a goal of developing novel inhibitors of these enzymes that are central targets in the treatment of hormone dependent cancers and in the role of electron transfer partners in controlling metabolic flux. We use a wide variety of biophysical and chemical tools in our research, including NMR, EPR, optical and Raman spectroscopies, rapid reaction methodologies such as stopped-flow, structural investigations via crystallography and x-ray scattering as well as high pressure and cryoenzymology.