Stroke is the third leading cause of death and the leading cause of chronic disability in the United States. In the past several decades, case series, case-control studies, and prospective cohort studies have successfully identified nonmodifiable risk markers for stroke, such as age, gender, race, ethnicity, heredity and several well-established modifiable risk factors for ischemic stroke. Hypertension, atrial fibrillation, other cardiac diseases, hyperlipidemia, diabetes, cigarette smoking, physical inactivity, carotid stenosis, and transient ischemic attack (TIA) are all potentially treatable conditions that predispose to stroke. Research on other putative stroke risk factors-including antiphospholipid antibodies, elevated homocysteine, alcohol, inflammation, and infection-is ongoing. Controlled trials have shown that stroke risk can be reduced by blood-pressure control, lipid-lowering agents, surgery for carotid stenosis, warfarin for atrial fibrillation, and antiplatelet agents. It is hoped that an improved understanding of stroke risk factors will reduce the future burden of stroke.