The angiographic changes associated with moyamoya are caused by numerous heterogeneous and pathologic processes. The disease process is characterized by progressive occlusion of one or both internal carotid arteries and their proximal branches. As a result of reduced blood flow in the anterior circulation, collateral circulation composed of small vessels develops near the apex of the carotid artery, meningeal vessels, and dural/basilar branches of the external carotid artery. The angiographic abnormalities are usually bilateral, with differences in severity between sides. The affected vessels and collateral vessels are generally maximally dilated; consequently, patients are vulnerable to ischemic injury with small changes in cerebral blood flow. Patients with associated conditions, such as neurofibromatosis type I, sickle cell disease, trisomy 21, or history of radiotherapy for intracranial tumor (i.e., optic glioma, craniopharyngioma, or pituitary tumor) are referred to as having moyamoya syndrome. Patients without associated risk factors are said to have moyamoya disease. Patients with unilateral disease—with or without associated risk factors—are identified as having moyamoya syndrome. In up to 40% of patients with unilateral moyamoya, contralateral disease may develop, particularly in those of younger age.