Vitamin D is a group of fat-soluble prohormones, the two major forms of which are vitamin D2 (or ergocalciferol) and vitamin D3 (or cholecalciferol). Vitamin D obtained from sun exposure, food, and supplements is biologically inert and must undergo two hydroxylation reactions to be activated in the body. This book presents research in the study of Vitamin D, including Vitamin D and thyroid hormones, Vitamin D from dolphin skins to clinical trials, Vitamin D3 and immunity to tuberculosis and Vitamin D deficiency. Also discussed herein is Vitamin D in the preclinical studies and clinical applications of renal cell carcinoma and new aspects of Vitamin D endocrinology.