Cytochrome P450 isozyme 3A4 (CYP3A4) is a major isozyme in the human liver and is known to metabolize a larger variety of xenobiotics and endogenous biochemicals. The identities of CYP3A4 substrates are summarized here. A total of 32 chemicals belonging to different structural classes have been evaluated and found to be substrates for CYP3A4. The metabolic pathways for these substrates include N-oxidation, C-oxidation, N-dealkylation, O-dealkylation, nitro-reduction, dehydration, and C-hydroxylation. While the major experimental system used to elucidate the role of CYP3A4 in the metabolic transformation of these substrates is the human liver microsome system, cultured human hepatocytes and yeast/cultured cells genetically engineered to express CYP3A4 are also employed by the different investigators. The common approaches to identify the role of CYP3A4 are also summarized, which include correlation of metabolic activity of the substrates studied with those for known CYP3A4-catalyzed substrates, correlation of activity with CYP3A4 content, inhibition of activity with CYP3A4 specific antibodies, inhibition of activity with known CYP3A4 substrates and inhibitors, induction of activity with CYP3A4 inducers and demonstration of activity with purified CYP3A4 enzyme.