Study to Assess the Safety and Effectiveness of the Penumbra System

Inclusion Criteria: * Clinical signs consistent with acute ischemic stroke * 18 to 79 years of age * Neurological deficit resulting in an NIH Stroke Scale (NIHSS) score \> 8 * TIMI 0 or TIMI I flow in vessels accessible to the Penumbra System * Signed informed consent * Present within 8 hours of stroke symptom onset; ineligible or refractory to intravenous tissue plasminogen activator (t-PA) therapy if presenting within 3 hours of symptom onset. Exclusion Criteria: * Evidence of rapidly improving neurological signs of stroke at time of enrollment * NIHSS \> 30 or coma * Females who are pregnant * Vessel tortuosity too difficult to allow endovascular access * Known hemorrhagic diathesis, coagulation deficiency, or on oral anticoagulant therapy with an International Normalized Ratio (INR) \> 3.0 * Partial thromboplastin time (PTT) greater than 2 times the lab normal * Admission platelets \< 30,000 * Pre-existing neurological or psychiatric disease that could confound the study results * Known severe allergy to contrast media * Uncontrolled hypertension * Computed tomography (CT) evidence of significant mass effect with a midline shift * CT reveals evidence of large hypodensity region \> 1/3 of the middle cerebral artery territory * CT reveals evidence of intracranial hemorrhage * CT reveals significant mass effect with midline shift * Angiographic evidence of an arterial stenosis proximal to the occlusion that could prevent thrombus removal * Angiographic evidence of preexisting arterial injury * Life expectancy of less than 90 days * Participation in another clinical investigation that could confound the evaluation of the study device.