A TP-binding cassette (ABC) multidrug transporters are cellular efflux pumps with broad and often widely overlapping substrate specificities. They can have a major impact on the pharmacokinetics and hence overall pharmacological behavior of many drugs. To study their separate roles and functional overlap, or complementarity, a collection of mice deficient in two or more ABC transporters has been generated. This review discusses recent findings obtained with these models, focusing on pharmacokinetic studies with a number of clinically relevant drugs. In addition, the characterization of these mice and some physiological aspects of ABC multidrug transporters are addressed.