Safety and Efficacy Study of the Medtronic CoreValve® System in the Treatment of Severe, Symptomatic Aortic Stenosis in Intermediate Risk Subjects Who Need Aortic Valve Replacement (SURTAVI).

Inclusion Criteria: * Subject must have co-morbidities such that Heart Team agrees predicted risk of operative mortality is ≥3% and \<15% at 30 days (Intermediate Clinical Risk classification). Heart team evaluation of clinical surgical mortality risk for each patient includes the calculated STS score for predicted risk of surgical mortality augmented by consideration of the overall clinical status and co-morbidities unmeasured by the STS risk calculation; * Heart Team unanimously agree on treatment proposal and eligibility for randomization\* based on their clinical judgement (including anatomy assessment, risk factors, etc.); * Subject has severe aortic stenosis presenting with; 1. Critical aortic valve area defined as an initial aortic valve area of ≤1.0cm2 or aortic valve area index \< 0.6cm2/m2 AND 2. Mean gradient \> 40mmHg or Vmax \> 4m/sec by resting echocardiogram or simultaneous pressure recordings at cardiac catherization \[or with dobutamine stress, if subject has left ventricular ejection fraction (LVEF) \<55%\] or velocity ratio \< 0.25; * Subject is symptomatic from his/her aortic valve stenosis, as demonstrated by New York Heart Association (NYHA) Functional Class II or greater; * Subject and the treating physician agree that the subject will return for all required post-procedure follow-up visits; * Subject meets the legal minimum age to provide informed consent based on local regulatory requirements; Exclusion Criteria: * Subject has refused surgical aortic valve replacement (SAVR) as a treatment option; (not applicable for Single Arm) * Any condition considered a contraindication for placement of a bioprosthetic valve (i.e., subject requires a mechanical valve); * A known hypersensitivity or contraindication to all anticoagulation/antiplatelet regimens (or inability to be anticoagulated for the index procedure), nitinol, or sensitivity to contrast media which cannot be adequately pre-medicated; * Blood dyscrasias as defined: leukopenia (WBC \<1000mm3), thrombocytopenia (platelet count \<50,000 cells/mm3), history of bleeding diathesis or coagulopathy; * Ongoing sepsis, including active endocarditis; * Any condition considered a contraindication to extracorporeal assistance; * Any percutaneous coronary or peripheral interventional procedure performed within 30 days prior to randomization\* (Subjects with recent placement of drug eluting stent(s) should be assessed for ability to safely proceed with SAVR within the protocol timeframe); * Symptomatic carotid or vertebral artery disease or successful treatment of carotid stenosis within six weeks of randomization\*; * Cardiogenic shock manifested by low cardiac output, vasopressor dependence, or mechanical hemodynamic support; * Recent (within 6 months of randomization\*) cerebrovascular accident (CVA) or transient ischemic attack (TIA); * Active gastrointestinal (GI) bleeding that would preclude anticoagulation; * Subject refuses a blood transfusion; * Severe dementia (resulting in either inability to provide informed consent for the trial/procedure, prevents independent lifestyle outside of a chronic care facility, or will fundamentally complicate rehabilitation from the procedure or compliance with follow-up visits); * Multivessel coronary artery disease with a Syntax score \>22 and/or unprotected left main coronary artery (Syntax score calculation is not required for patients with history of previous revascularization if repeat revascularization is not planned); * Estimated life expectancy of less than 24 months due to associated non-cardiac comorbid conditions; * Other medical, social, or psychological conditions that in the opinion of the Investigator precludes the subject from appropriate consent or adherence to the protocol required follow-up exams; * Currently participating in an investigational drug or another device trial (excluding registries); * Evidence of an acute myocardial infarction ≤30 days before the index procedure; * Need for emergency surgery for any reason; * True porcelain aorta (i.e. Heart Team agrees the aorta is not clampable for SAVR); * Extensive mediastinal radiation; * Liver failure (Child-C); * Reduced ventricular function with left ventricular ejection fraction (LVEF) \<20% as measured by resting echocardiogram; * Uncontrolled atrial fibrillation (e.g. resting heart rate \> 120 bpm); * Pregnancy or intent to become pregnant prior to completion of all protocol follow-up requirements; * End stage renal disease requiring chronic dialysis or creatinine clearance \< 20 cc/min; * Pulmonary Hypertension (systolic pressure\> 80mmHg); * Severe Chronic Obstructive Pulmonary Disease (COPD) demonstrated by Forced Expiratory Volume (FEV1) \< 750cc; * Frailty assessments identify: 1. Subject is \< 80 years of age and three or more of the following apply 2. Subject is ≥ 80 years of age and two or more of the following apply * Wheelchair bound * Resides in an institutional care facility (e.g., nursing home, skilled care center) * Body Mass Index \< 20 kg/m2 * Grip Strength \< 16 kg * Katz Index Score ≤ 4 * Albumin \< 3.5 g/dL; * Marfan syndrome or other known connective tissue disease that would necessitate aortic root replacement/intervention; (Not applicable for Single Arm) Note: Additional anatomical and vascular exclusion criteria may apply. Note: \* For purposes of the single arm phase of the trial, "randomization" will refer to trial enrollment.