The bile acids as principal end products of cholesterol metabolism occupy a focal position in our understanding of the role of steroids in bio logical systems. The biogenesis of bile acids from cholesterol in higher ani mals, and their functions in regulating sterol metabolism and in gastrointestinal physiology have been elucidated by the development of elegant methodo logical approaches during the last two decades. The molecular pleomorphism exhibited by the bile acids and bile alcohols in the animal kingdom is a classic example of their role in biochemical evolution. The total story of the bile acids, their chemistry, their role in normal and abnormal physiological processes, and their significance in biochemical evolution has never been available in the form of a comprehensive treatise written in the words of those who have contributed to the development of our knowledge in this area. The Bile Acids, in two volumes, will serve to fill this void, and will also bring together information which will prove in valuable to both the biochemist and the medical scientist. We wish to thank Mrs. Sally Wiseman and Mrs. Lillian Haas for their invaluable assistance with the editing of the manuscripts. This work was supported in part by grants AM-02131, General Research Support SS0-1- FR-05479 (P.P.N.), HE-03299, HE-05209, and a National Heart Institute Research Career Award (D.K. ), K6-HE-734, from the National Institutes of Health, United States Public Health Service. P.P.N. Baltimore, Maryland D.K.