Low vitamin D levels are associated with cardiovascular disease and its precursors, hypertension, and diabetes mellitus. In this chapter, we review mechanistic studies in animals and tissue culture which suggest the role of vitamin D in controlling the renin-angiotensin-aldosterone system. Multiple observational studies in humans have shown an association between low vitamin D levels and cardiovascular disease and hypertension and diabetes mellitus. Mendelian randomization studies and randomized clinical trials have shown mixed results. Current ongoing clinical trials will clarify whether the vitamin D and cardiovascular disease connection is a causal one.