Moyamoya disease is characterized by bilateral stenosis or occlusion of distal internal carotid artery (ICA) bifurcation including its proximal branches and abnormal vascular network (moyamoya vessel, MMV) in the vicinity of the arterial occlusions. It is the most common pediatric cerebrovascular disease in Eastern Asia, particularly in Korea and Japan. The etiology is still unknown, but much about the pathology from autopsies, factors involved in its pathogenesis, and its genetics have been studied and reported. It may cause ischemic attacks or cerebral infarctions in children and cerebral hemorrhage in adults. Because of its aggressive clinical course in very young children, the need for early detection and treatment has been recognized. Magnetic resonance imaging (MRI)/MR angiography (MRA), cerebral hemodynamic studies, and cerebral angiography are used for the diagnosis. The treatment basically focuses on prevention of further ischemia and infarction through revascularization. Technically, direct and indirect bypass methods are used. The treatment strategy needs to be individualized in each patient. Outcomes of revascularization procedures are excellent in preventing transient ischemic attacks (TIAs) in most patients.