Endothelial dysfunction, a term used to describe both the physical damage and dysregulated physiology of this endothelial lining, is an increasingly recognized pathophysiological state shared by many cardiovascular diseases. Historically, the role of endothelial dysfunction in atrial fibrillation (AF) was thought to be limited to mediating atrial thromboembolism. However, there is emerging evidence that endothelial dysfunction both promotes and maintains atrial arrhythmic substrate, predicts adverse outcomes, and identifies patients at high risk of recurrence following cardioversion and ablation therapy. Treatments targeted at improving endothelial function also represent a promising new therapeutic paradigm in AF. This review summarizes the current understanding of endothelial function in AF.