Vitamin D is widely known for its important role in bone health. More recent evidence suggests that vitamin D may also play a protective role in many chronic conditions, including cancer, autoimmune, kidney, and cardiovascular diseases (CVDs). Observational studies have associated low vitamin D levels with CVD risk factors, including hypertension, hyperlipidemia, diabetes, and metabolic syndrome, as well as with cardiovascular events, including stroke, myocardial infarction, and congestive heart failure. Much less evidence is available from clinical trials of vitamin D supplementation. It remains to be determined whether the vitamin D–CVD relationship is causal, and what dosing of vitamin D supplementation would be adequate for prevention. Large-scale randomized controlled trials with adequate vitamin D dosing are needed before treatment strategies can be implemented. The purpose of this article is to review the scientific evidence linking vitamin D deficiency with CVD, including explanations of potential biologic mechanisms.