Vitamin D deficiency has become a global health concern, affecting over a billion people. In India, studies indicate that nearly 76% of the population are affected without receiving any significant attention yet remains a crucial health issue. Initially identified for its role in calcium homeostasis and skeletal integrity, vitamin D, also popularly known as the “sunshine vitamin D,” is a steroid hormone that has emerged as a critical regulator of multiple systems, including immunity, metabolismMetabolism, cardiovascular health and neuroprotection. Vitamin D exists in two forms—vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D3 is synthesized in the skin on exposure to ultraviolet radiations. It further undergoes hydroxylation in the liver and kidneys to for calcitriol (1, 25-dihydroxyvitamin D), which binds to vitamin D receptors in the target tissues, influencing gene expression and cellular function. Apart from promoting calcium and phosphate absorption, it also modulates the human immune responses, suppresses pro-inflammatory cytokines, and enhances the antimicrobial defense mechanisms. It is also proved that vitamin D deficiency is linked to many autoimmune diseases, diabetes, and certain types of cancers. The chapter deals with the physiological functions of vitamin D, its synthesis, metabolism, mechanism of action, effects of deficiency, and its supplementation and therapeutic approaches in detail. There is also growing evidence supporting vitamin D’s role in immunity, cardiovascular health, and neuroprotection. Vitamin D deficiency can be corrected through diet, sun exposure, and supplementation which makes it a candidate for preventative medicine. This chapter seeks to highlight the disparity between available clinical evidence and public consciousness in order to illustrate the importance of a variety of proactive measures.