Cytochrome P450 (CYP) proteins, named for their absorption band at 450 nm, are one of the largest superfamilies of enzyme proteins. CYP genes are found in almost all organisms, with a particularly high number in plants. Despite their diverse amino-acid sequences, CYPs share a conserved structural fold and heme-thiolate properties. CYPs catalyze a wide range of reactions, including hydroxylation, dealkylation, dehydration, and carbon-carbon bond cleavage, often in the context of xenobiotic metabolism and drug metabolism. In plants, chemical defense is a major driver of CYP diversification. CYPs are classified into four classes based on their electron transfer mechanisms and are found in various subcellular locations, including the endoplasmic reticulum and mitochondria. The evolutionary history of CYP genes involves extensive gene duplications and losses, with the superfamily originating in prokaryotes. The functional diversity of CYPs is reflected in their roles in drug metabolism, xenobiotic detoxification, and the biosynthesis of natural products. Key unresolved questions include the physiological functions of newly discovered CYPs and the molecular basis of receptor-mediated transcriptional activation.Cytochrome P450 (CYP) proteins, named for their absorption band at 450 nm, are one of the largest superfamilies of enzyme proteins. CYP genes are found in almost all organisms, with a particularly high number in plants. Despite their diverse amino-acid sequences, CYPs share a conserved structural fold and heme-thiolate properties. CYPs catalyze a wide range of reactions, including hydroxylation, dealkylation, dehydration, and carbon-carbon bond cleavage, often in the context of xenobiotic metabolism and drug metabolism. In plants, chemical defense is a major driver of CYP diversification. CYPs are classified into four classes based on their electron transfer mechanisms and are found in various subcellular locations, including the endoplasmic reticulum and mitochondria. The evolutionary history of CYP genes involves extensive gene duplications and losses, with the superfamily originating in prokaryotes. The functional diversity of CYPs is reflected in their roles in drug metabolism, xenobiotic detoxification, and the biosynthesis of natural products. Key unresolved questions include the physiological functions of newly discovered CYPs and the molecular basis of receptor-mediated transcriptional activation.