Vitamin D is a fat-soluble vitamin that helps the human body absorb and retain phosphorus and calcium, which are essential for development of bones. It is important for human health. It can be acquired through food, sunlight, and nutritional supplements. In human body, its consumption promotes calcium absorption and calcium homeostasis. This makes it an important nutrient for maintaining bone health. There are two main types of vitamin D, namely, vitamin D2 and vitamin D3. Vitamin D2 is also known as ergocalciferol; and vitamin D3 is also known as cholecalciferol. Vitamin D is also crucial for metabolism of phosphorus as it controls intestinal absorption and improves reabsorption of the mineral. The lack of vitamin D may cause severe health problems such as osteoporosis, rickets, and osteomalacia. This book explores the functions and effects of vitamin D in human health. It presents researches and studies performed by experts across the globe. Those in search of information to further their knowledge will be greatly assisted by this book.