Recent evidence suggests a number of mechanisms whereby vitamin D may positively influence the pathophysiology of heart failure. These include actions on the renin-angiotensin system, calcium handling, reduction of proinflammatory cytokines, and improvements in endothelial function and blood pressure. Observational data suggest that low vitamin D levels are common in patients with heart failure and are associated with worse exercise capacity and natriuretic peptide levels. Little interventional data are currently available, but evidence to date does not support vitamin D supplementation, even in patients with low vitamin D levels. Further studies are needed to establish whether larger doses of vitamin D given over a longer period of time can reduce symptoms, hospitalization, and mortality in heart failure.