The broad outlines of the physical chemistry of cholesterol solubilization are well understood, but minor components of native biles probably alter the equilibrium solubility of cholesterol in bile and, more importantly, profoundly affect the kinetics of cholesterol crystal formation. The physical chemistry of UCB solubilization is much less clear; two decades after the limits of cholesterol solubilization in bile were reported, we still cannot provide accurate information on the limits of UCB solubility in bile. Although UCB is predominantly solubilized by the same biliary aggregates as cholesterol, the ionic nature of UCB implies that other, quantitatively minor biliary components profoundly alter UCB solubilization. Understanding UCB solubilization in bile and the prevention of black pigment stone formation await further studies. An understanding of the physical chemistry of cholesterol and UCB, both as solubilized in bile and as precipitated in gallstones, is basic to an understanding of the pathogenesis of gallstone disease, as well as current and potential chemical methods of gallstone dissolution and prevention.