The three lipids in bile, cholesterol, lecithin, and bile salts (about 90 percent of the dry weight of normal gallbladder bile) are amphipathic substances having both hydrophobic and hydrophilic functional groups. Knowledge of the physicochemical factors of gallstone formation (especially cholesterol stones) has increased in the past two decades. The absolute amount of cholesterol supersaturation determines the extent of cholesterol precipitation. The ionic strength of the bile and the types of bile salts present are minor factors, whereas the ratios of bile salts to lecithin at a particular concentration of total lipids are the major factors contributing to gallstone production. Bile acids (salts) form micelles which allow the lecithin and cholesterol to dissolve within the micelles. Thus the administration of bile acids allows for non-invasive dissolution of some cholesterol gallstones. Additional important risk factors are genetic and ethnic, sex (females predominate), obesity, diet (in contrast to animal protein and more refined carbohydrate diets, there is less lithogenicity with diets containing plant protein and unrefined carbohydrates), certain diseases, and drug therapy. Pigment stones make up the majority of radiopaque stones and are predominant in the Orient; they are seen in certain diseases and in infections of the biliary tree.