Obstructive sleep apnea (OSA) is a very common condition in patients with stroke and is found in over half of stroke patients. There is a complex relationship between OSA and stroke, attributable to shared risk factors. There are numerous mechanisms by which OSA may contribute to increased stroke risk, including promotion of atherosclerosis, hypercoagulability, and adverse effects on cerebral hemodynamics. Obstructive sleep apnea is also a risk factor for hypertension, and likely for atrial fibrillation and diabetes, conditions that in turn are risk factors for stroke. OSA is also associated with poor outcomes following stroke. Further epidemiological studies are needed to assess the relationship between OSA and stroke better. Clinical trials using continuous positive airway pressure as a treatment for OSA in stroke patients are needed to determine whether treatment of this condition alters outcome following stroke.