This chapter provides detail description of Vitamin D. Chapter has 14 well-illustrated diagrams and pictures. Biochemistry and metabolism of vitamin D, sources, requirement, functions of vitamin D, variations in vitamin D status, interaction with other nutrients, epidemiology, vitamin D deficiency, vitamin D deficiency, rickets and other conditions are given in detail. 2 tables are included to highlight biochemical changes in rickets and laboratory findings in disorders causing rickets. 3 boxes are added to depict predisposing factors for development of rickets, etiological classification of rickets and signs of rickets. 13 references are given at the end of the chapter for further reading.