Numerous sources of human heterogeneity affect biotransformation of compounds. Cytochrome P450, the primary oxidative pathway of drug metabolism, is the dominant phase I oxidative system metabolizing, to some degree, most of the drugs used clinically in humans. The P450 pathway is a major site of drug-drug, drug-diet, and drug-disease/condition interactions. Functional variability in this system can have pronounced consequences in suboptimal therapeutic response or enhanced toxicity. Methods for cataloguing specific P450 enzymes are being developed and their identification will promote rational drug development, more efficient clinical trial evaluation, and improved therapeutic approaches to patients requiring special consideration. These methods will facilitate the study of the impact of pubertal development on function in this system.