Vitamin D is a fat-soluble secosteroid and helps in calcium homeostasis in the body. Vitamin D receptor is present in different types of cells within the body, and through this receptor, vitamin D influences several intracellular pathways, thus exerting its metabolic function. Apart from the musculoskeletal effect, vitamin D has been attributed to participate in cellular functions of several non-skeletal systems. Due to its influence on several systems within the body, it helps to maintain the proper balance required for a healthy physiological support. Deficiency in this micronutrient may be a risk factor for several interconnected physiological imbalances. In this chapter, we focus on the source, metabolism and effect of vitamin D in the human body with special reference to its influence on clinical aspect.