The overall goal of the study is to define the population of patients with symptomatic VBD at highest risk of recurrent ischemic events. The information gained can significantly impact the selection criteria and likelihood for success of future clinical trials aimed at assessing the efficacy of endovascular or surgical interventions for the treatment of VBD. Moreover, the ability to define a low risk population in whom the risks of expensive invasive interventions would be unnecessary will have an equally important impact on the management of the disease both from a clinical and cost perspective. Data regarding the hemodynamic effects and changes over time of vertebrobasilar occlusive disease may also enhance our understanding of the basic pathophysiology and mechanisms of stroke in this morbid disease entity.