Amphiphatic bile salts are the main product of cholesterol metabolism. Their structure determines their biological and physicochemical properties which in turn are related to their multifaceted biological behavior. Their main biological function is to solubilize dietary lipids and thus greatly accelerate their absorption. The efficient performance of this function is facilitated by the concentrating effect of the gallbladder upon the bile secreted by the liver, the solubility of the bile salts under conditions prevailing in the small intestine, and their ability to form micelles which are powerful solubilizers for fatty acids and monoglycerides. In addition, the resistance of bile salts to digestive enzymes and to absorption by the mucosa of the upper intestine, combined with their rapid absorption in the lower intestine and ready resecretion by the liver, makes possible the repeated reuse of most of the bile salt molecules, thus reducing the metabolic burden of this operation. The unique suitability of natural bile salts to perform their functions becomes apparent when unnatural analogues, differing in structure, are prepared and their physicochemical and biological properties are studied.