The decision to operate in patients with aortic stenosis is based on the presence of symptoms and hemodynamically significant valvular obstruction. The strengths and limitations of cardiac catheterization and Doppler echocar-diography are compared, and the concept of “critical” aortic stenosis is discussed. The recommendation of aortic valve replacement must take into account the symptom status, the hemodynamic significance of the lesion, and the size and type of valve to be implanted.