The role of individual hepatic cytochrome P450 (CYP) enzymes in drug metabolism and the factors that modulate CYP activity are becoming increasingly well understood. These advances have resulted in a better understanding of drug-drug and drug- food interactions and an enhanced capacity to predict drug interactions that may occur with new drugs. This final article in the series describes the issues and principles that are important in identifying and assessing drug interactions that involve CYP enzymes.