A fundamental aspect of the practice of a medical or surgical specialty is clinical decision making. This process presumes a careful comparison of the risks of various treatment options and the risks of the natural history of the patient's disorder. A multitude of factors enter into an accurate assessment of the risks involved in any surgical or medical therapy. In the specialty of vascular neurosurgery, these risks are often of great consequence. Potential complications underscore the importance of the other side of the clinical decision-making equation, namely, the natural history of the patient's affliction. Our knowledge of the natural history of intracranial vascular anomalies is incomplete. Much, however, is known of the outlook for patients harboring intracranial vascular malformations and aneurysms. It is of paramount importance that this knowledge be reviewed periodically so that the most accurate information is used when one consults an individual patient on the most appropriate course of therapy.