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. Calcitriol is the active form of vitamin D found in the body. Its major role is to increase the flow of calcium into the bloodstream, by promoting absorption of calcium and phosphorus from food in the intestines, and reabsorption of calcium in the kidneys. It is also necessary for bone growth and bone remodelling by osteoblasts and osteoclasts. This book examines the biochemical components of vitamin D and the nutritional benefits it offers the human body.