The purpose of the study is to evaluate the safety and clinical performance of the Engager Transcatheter Aortic Valve Implantation System in patients with severe aortic valve stenosis who are at high risk for surgical valve replacement.

Inclusion Criteria:

1. Severe aortic stenosis defined as one or more of the following by Doppler echocardiography: mean gradient > 40 mmHg; max velocity > 4m/s; aortic valve area ≤ 0.8 cm2.
2. Symptoms related to aortic valve disease, and NYHA Functional Class II or greater.
3. Logistic EuroSCORE predicted risk for mortality of ≥20%, or comorbidity judged by the investigator to pose an absolute or relative contraindication for conventional aortic valve replacement.
4. Patient is indicated for aortic valve implantation with a biological prosthesis (tissue valve) in accordance with the 2007 European Society of Cardiology (ESC) Guidelines for management of valvular heart disease.
5. Age ≥ 18 years.
6. Echocardiographically determined aortic annulus diameter of ≥19 mm and ≤ 26 mm in a long-axis view.

Exclusion Criteria:

1. Congenital unicuspid or bicuspid aortic valve.
2. Severe eccentricity of calcification, defined as calcium deposits larger than 6mm in diameter.
3. Echocardiographic evidence of intracardiac mass, thrombus, or vegetation.
4. Left ventricular ejection fraction < 25%, as determined by contrast ventriculography, or echocardiography or radionuclide angiography if contrast ventriculography not available
5. Hypertrophic obstructive cardiomyopathy.
6. Patients with life expectancy less than 12 months due to an underlying non-cardiac comorbid disease.
7. Known hypersensitivity or contraindication that cannot be adequately controlled with pre-medication to any study medication or material, such as contrast medium or Nitinol.
8. Sepsis or acute endocarditis.
9. Blood dyscrasia such as acute anemia, leucopenia, or thrombocytopenia; bleeding diathesis, or coagulopathy.
10. Renal insufficiency assessed by creatinine > 2.5 mg/dl and/or end stage renal disease requiring chronic dialysis.
11. Active peptic ulcer or GI bleeding within 3 months from the planned index procedure.
12. Untreated clinically significant coronary artery disease requiring revascularization.
13. Cardiogenic shock, suspected cardiogenic shock, or hemodynamic instability requiring inotropic support or mechanical heart assistance.
14. Significant aortic disease, including abdominal and thoracic aortic aneurysm, defined as maximal luminal diameter of 5 cm or greater.
15. Need for emergency surgery, cardiac or non-cardiac.
16. History of myocardial infarction in the last 6 weeks.
17. History of TIA or CVA in the last 6 months.
18. Therapeutic invasive cardiac procedure, with the exception of aortic balloon valvuloplasty, performed within 30 days of the planned date of valve implantation, or 6 months in the case of drug-eluting stents.
19. Pre-existing prosthetic heart valve or prosthetic ring in any position.
20. Mitral regurgitation greater than 2+ by angiography or moderate by echocardiography.
21. Patient refuses a blood transfusion.
22. Patient is currently enrolled in another investigational device or drug trial that may influence the outcome of this trial.