The xenobiotic-metabolizing cytochrome P450s are found in the CYP1, CYP2, CYP3 and CYP4 subfamilies (Gonzalez, 1988). These P450s have been extensively studied in a number of organisms including bacteria, yeast, fish, chickens, rats, mice, rabbits and humans. The P450 genes are under control of diverse regulatory networks that govern their expression. These networks include foreign chemical receptors that activate gene expression through binding to inducible enhancer elements and tissue-enriched transcription factors that control constitutive expression. This review focuses on recent developments on the molecular mechanisms of P450 gene control.