This issue of Endocrinology and Metabolism Clinics, guest edited by Drs. J. Chris Gallagher and Daniel Bikle, is devoted to Vitamin D. Articles in this issue include: The Molecular and Cellular Mechanisms of Action of Vitamin D, Worldwide Review of Serum 25OHD Status; RDA Update (7 Years Post IOM); The Use of Vitamin D Metabolites and Analogs in the Treatment of Chronic Kidney Disease; Effect of Vitamin D on Falls and Physical Performance; Vitamin D Effect on BMD and Fractures; Bariatric Surgery and Vitamin D Metabolism, Role of Vitamin D in the Prevention and Treatment of Cancer; Role of Vitamin D in Cardiovascular Disease; Regulation of Immune Function by Vitamin D and its Use in Diseases of Immunity; Genetic Diseases of Vitamin D Metabolizing Enzymes; Measurement of Vitamin D Metabolites: Clinical Issues; The Free 25OHD Hypothesis, and Ethnic/Racial Differences in Vitamin D Levels: Physiologic or Pathologic.