Our understanding of calcific aortic stenosis has changed dramatically during the past 3 decades, with the concept of a "mechanical" disease of aging now replaced by the concept of an active disease process at the tissue level that may be amenable to medical therapy. The ability of echocardiography to provide early diagnosis and an accurate measurement of disease severity has increased our knowledge of the natural history of this disease process and allows us to follow individual patients over time, long before valve replacement is needed. We now recognize that even mild symptoms are an indication for valve replacement when severe obstruction is present. This review discusses the optimal approach to measurement of disease severity, the presymptomatic disease course, and the underlying causes of calcific valve disease, followed by a summary of clinical trials of medical therapy and the current indications and choices for valve replacement.