Vitamin D is a vitamin which increases the absorption of phosphorus and calcium from the intestine and stimulates their deposition into bones. The two most important types of vitamin D are vitamin D3 and vitamin D2. Vitamin D3 is obtained by exposure to sunlight and dietary sources such as fish or other supplements. Whereas, few foods except mushrooms consist of vitamin D2, and primarily it is found in fortified foods and supplements. They both are varied by their side chain structure. The side chain of vitamin D3 consists of a broken ring; whereas vitamin D2 consists of a double bond among carbons 22 and 23 along with methyl group on carbon 24 in its broken ring. Vitamin D is essential for bone health of human beings. It is also helpful in decreasing the risk of chronic diseases such as cardiovascular diseases, autoimmune diseases and cancer. This book aims to understand the physiology, molecular biology and clinical applications of vitamin D. It is a valuable compilation of topics, ranging from the basic to the most complex studies on this topic. This book is appropriate for students seeking detailed information in this area of study as well as for experts.