Aortic stenosis (AS) is the most commonly acquired valvular heart disease in the Western world (2-7% of the population aged >65 years) and the mortality for untreated symptomatic severe AS is up to 50-60% at 2 years in high-risk patients. This review summarizes laboratory and recent clinical investigations concerning AS and current best treatment. Particular emphasis will be placed on pathophysiology and on the concept of afterload mismatch and preload reserve in which left ventricular function is proportionately matched to level of left ventricular pressure, mainly because such studies carry important implications for both treatment with transcatheter aortic valve implantation (TAVI) and prognosis. Surgical aortic valve replacement is currently the gold-standard treatment for patients with severe symptomatic AS. Without surgery, the prognosis is extremely poor, with a 3-year survival rate <30%. However, 33% of all patients aged ≥75 years with severe AS are declined for surgery. TAVI was recently introduced as a new therapeutic option for patients with AS, so the current indications, the technical differences between 2 different transcatheter aortic valves and the clinical available data will be also examined in detail. (Circ J 2011; 75: 11-19)