The ATP-binding cassette transporter P-glycoprotein is now recognized as an important determinant for disposition of multiple drugs. The use of P-glycoprotein-expressing cell lines, the generation of P-glycoprotein knockout mice as well as studies in animals and humans contributed to a better understanding on the role of active transport processes for drug disposition. P-glycoprotein is located in tissues with excretory function such as intestine, liver and kidney. Moreover, due to its expression in important blood-tissue barriers (blood-brain and blood-testis barriers), in lymphocytes and in placenta it limits tissue penetration of its substrates. Induction and inhibition of P-glycoprotein have now been identified as important underlying mechanisms of drug interactions in humans. Using selected examples, this review summarizes currently available data on the impact of P-glycoprotein for bioavailability of drugs, drug interactions and drug effects.