Vitamin D is a secosteroid, fat-soluble vitamin and a hormone precursor that plays an important role in bone metabolism and has certain anti-inflammatory and immune-modulating properties as well. It appears to have an effect on numerous diseases and disorders, including osteoporosis, chronic musculoskeletal pain, diabetes, multiple sclerosis, cardiovascular disease and cancers of the breast, prostate and colon. Although it is a well-known fact that combination of vitamin D and calcium is necessary to maintain bone density as people age, vitamin D may also be an independent risk factor for falls among the elderly. This book introduces the importance of vitamin D and its overall deficiency in human life. The book contains nine chapters and a commentary in which the authors try to summarize some of the most interesting and important subjects in which vitamin D plays a major role.