The worsening worldwide trend toward nutritional insufficiency and the emerging knowledge of the nonhormonal actions of vitamin D and its metabolites have increased interest in the synthesis, metabolism, and action of vitamin D. Vitamin D deficiency has been linked with hypertension, myocardial infarction, and stroke, as well as other cardiovascular-related diseases, such as diabetes, congestive heart failure, peripheral vascular disease, atherosclerosis, and endothelial dysfunction. This review discusses the physiology and definition of vitamin D deficiency, evaluates the worldwide prevalence of vitamin D deficiency, and discusses recent evidence for the association between hypovitaminosis D and cardiovascular disease. Few randomized, controlled trials have evaluated the effect of vitamin D replacement on cardiovascular outcomes, and the results have been inconclusive or contradictory. Carefully designed randomized, controlled trials are essential to evaluate the role of vitamin D supplementation in reducing cardiovascular disease.