Many advances in vitamin D physiology and biochemistry have been made in recent years. Vitamin D metabolites and analogs have found increasing application in clinical medicine. The purpose of this text is to review what is known about vitamin D physiology and draw attention to areas of vitamin D research that have changed within the last 2-3 years. Additionally, information concerning clinical aspects of vitamin D is also presented. More than 40 scientists have generously contributed chapters to this text; I thank them for their efforts. As might be expected, not everyone has the same point of view. Finally, I would like to acknowledge the secretarial and editorial efforts of Mrs. Cheryl Collins without whom this book would not have been completed. CONTRIBUTORS ETSUKO ABE, Department of Biochemistry, School of Dentistry, Showa University, 1-5-8, Hatanodai, Shinagawa-KU, Tokyo 142, Japan DAVID J. BAYLINK, Department of Medicine, Loma Linda University, Loma Linda, CA, and Pettis Veterans Hospital, 11201 Benton Street, Loma Linda, CA, 92357, USA NORMAN H. BELL, Department of Medicine, Medical University of South Carolina and Veterans Administration Medical Center, 109 Bee Street, Charleston, SC, 29403, USA WARNER M. BURCH, Jr. , Departments of Medicine and Physiology, Duke Univer­ sity Medical Center, Durham, NC, 27710, USA DAVID V. COHN, ICCRH, Inc. , 1238 Wyncrest Court, Arden Hills, MN, 55112, USA ROBERT A. CORRADINO, Department of Physiology, New York State College of Veterinary Medicine, Cornell University, 720 VRT, Ithaca, NY, 14853, USA HECTOR F.