This chapter provides a review of the pharmacogenetics of mem brane transporters, including adenosine triphosphate-binding cassette (ABC) transporters and organic anion transporting proteins (OATPs). Membrane trans porters are heavily involved in drug disposition by actively transporting substrate drugs between organs and tissues. As such, polymorphisms in the genes encoding these proteins may have a significant effect on the absorption, distribution, metabo lism, and excretion of compounds. The techniques used to identify substrates and inhibitors of these proteins and subsequently assess the effect of genetic mutation on transport, both in vitro and in vivo, are outlined and discussed. Finally, studies linking transporter genotype with clinical outcomes are discussed.