Stroke is a common cause of death in South Korea. The etiology of stroke affects prognosis, outcome, and management. Trials of therapies for patients with acute stroke should include measurement of responses as influenced by the subtypes of stroke. Stroke can be classified into ischemic and hemorrhagic stroke. Ischemic stroke classification denotes five subtypes of ischemic stroke : ① large-artery atheromatous disease, ② cardioembolism, ③ lacunes, ④ stroke of other causes, and ⑤ transient ischemic attack. Hemorrhagic stroke can be classified into ① intracerebral hemorrhage, ② subarachnoid hemorrhage, and ③ other hemorrhages. Stroke is a syndrome with various etiologies, and proper classification is important for the treatment and prognosis. Risk factors for stroke such as hypertension, heart disease, smoking, diabetes, and hypercholesterolemia should be controlled properly. Clinical trials testing new treatment for acute stroke are on-going worldwide, and it is appropriate to standardize the classification of stroke.