Bile, freshly synthesized by the liver, is an isosmotic solution containing principally sodium, conjugated bile acid anions, chloride, bicarbonate, and bilirubin, as well as lesser amounts of other mono-and divalent ions, cholesterol, phospholipid, and various proteins. Hepatic bile is stored in the gall bladder, where its volume is reduced by the absorption of an essentially isotonic sodium chloride-bicarbonate solution; the concentration of such biliary components as bile salts, bilirubin, and cholesterol, to which the gall bladder wall is essentially impermeable, may increase lO-fold or more. Recent developments in our knowledge of the mechanisms whereby the mucosal epithelium of the gall bladder is able to accomplish this concentration is the subject of the present review. No attempt has been made to cite every recent work in this field; rather, those investigations which have elucidated new and unique aspects of the over-all absorptive process have been emphasized, and in some cases, discussed in detail.