This chapter provides detail description of low-gradient, severe aortic stenosis with depressed and preserved ejection fraction. Myocardial response to chronic aortic stenosis, high-flow, high-gradient aortic stenosis with normal ejection fraction, low-flow, low-gradient aortic stenosis with low ejection fraction, low-flow, low-gradient aortic stenosis with normal ejection fraction, mechanisms behind PLFLG-AS, role of surgical aortic valve replacement (SAVR) in aortic stenosis are described in detail. The chapter has 4 well-illustrated diagrams and images. One table is included to highlight typical characteristics of three main entities of severe aortic stenosis. Two boxes are added to definitions of aortic jet velocity, mean pressure gradient, valve area, and Dobutamine stress echocardiography. 49 references are included at the end of the chapter to facilitate further reading.