Vitamin D deficiency and diabetes mellitus are two common conditions and they are widely prevalent across all ages, races, geographical regions, and socioeconomic conditions. Epidemiologic studies have shown association of vitamin D deficiency and increased risk of chronic diseases, such as cancer, cardiovascular disease, type 2 diabetes, and autoimmune diseases, such as multiple sclerosis and type 1 diabetes mellitus. The identification of 1,25(OH)2D receptors and 1-α-hydroxilase expression in pancreatic beta cells, in cells of the immune system, and in various others tissues, besides the bone system support the role of vitamin D in the pathogenesis of type 2 diabetes. Observational studies have revealed an association between 25(OH) D deficiency and the prevalence of type 1 diabetes in children and adolescents. This review will focus on the concept of vitamin D deficiency, its prevalence, and its role in the pathogenesis and risk of diabetes mellitus and cardiovascular diseases.