The organic anion transporting proteins (OATPs) belong to the SLC gene superfamily of transporters and are 12 trans-membrane domain glycoproteins expressed in various epithelial cells. Some OATPs are expressed in a single organ, while others occur ubiquitously. The functionally characterized members of the OATPs mediate sodium-independent transport of a variety of structurally independent, mainly amphipathic organic compounds, including bile salts, hormones and their conjugates, toxins, and various drugs. Uptake transporters (OATPs, NTCP, OCT1, and OAT2) are localized in the basolateral membrane. These transporters mediate the uptake of substrates into the liver from the circulation. OATP1B1 and OATP1B3 are liver specific and show broad substrate specificity (statins, rifampicin, and telmisartan). Inhibition of OATP-mediated uptake of several statins by cyclosporin A and rifampicin causes clinically significant DDIs. Some organic anions are excreted preferentially via the bile and some via the urine. In the human, small organic anions of molecular mass less than 500 Da are excreted exclusively by the kidney while bigger anions are preferentially excreted into bile. Conjugation with glucuronic acid or glutathione serves to increase the molecular mass of a substance by 176 Da and 306 Da, respectively and conjugation may therefore increase the likelihood of secretion of the anion into bile.