The organic solute and steroid transporter, Ost alpha-Ost beta, is an unusual heteromeric carrier that appears to play a central role in the transport of bile acids, conjugated steroids, and structurally-related molecules across the basolateral membrane of many epithelial cells. The transporter’s substrate specificity, transport mechanism, tissue distribution, subcellular localization, transcriptional regulation, as well as the phenotype of the recently characterized Ost alpha-deficient mice all strongly support this model. Ost alpha-Ost beta is composed of a predicted 340-amino acid, 7-transmembrane (TM) domain protein (Ost alpha) and a putative 128-amino acid, single-TM domain polypeptide (Ost beta). Heterodimerization of the two subunits increases the stability of the individual proteins, facilitates their post-translational modifications, and is required for delivery of the functional transport complex to the plasma membrane. Ost alpha and Ost beta are expressed in nearly all human tissues that have been examined, but are most abundant in the small intestine, kidney, liver, testis, adrenal gland and other steroidogenic tissues. Ost alpha-Ost beta substrates include bile acids, steroids (estrone 3-sulfate, dehydroepiandrosterone 3-sulfate, and digoxin), and prostaglandin E2, indicating a role of Ost alpha-Ost beta in the disposition of key cellular metabolites and signaling molecules. Transport occurs by a facilitated diffusion mechanism, and thus Ost alpha-Ost beta can mediate cellular efflux or uptake depending on that substrate’s electrochemical gradient. Additional strong evidence for a role of Ost alpha-Ost beta in sterol homeostasis was provided by recent studies in Ost alpha-deficient mice. These mice display a marked defect in intestinal bile acid and conjugated steroid absorption; a decrease in bile acid pool size and serum bile acid levels; altered intestinal, hepatic and renal disposition of known substrates of the transporter; and altered serum triglyceride, cholesterol, and glucose levels. Taken together, these observations indicate that Ost alpha-Ost beta is essential for bile acid and sterol disposition, and suggest that the carrier may be involved in human conditions related to imbalances in bile acid or lipid homeostasis.