Aortic stenosis is a common valvular lesion encountered in the ICU. Aortic stenosis restricts outflow from the left ventricle to the aorta, which can independently cause or contribute to shock and heart failure. Correct diagnosis and classification require an understanding of the causes of aortic stenosis and hemodynamic principles used to classify disease severity. Aortic stenosis is most commonly caused by age-related valvular calcification, but genetic, inflammatory, and infectious causes exist. Proper diagnosis of aortic stenosis etiology can offer insights into vascular pathology and systemic disease that may contribute to critical illness. Similarly, correct classification of aortic stenosis severity can help providers better manage patient hemodynamics and heart failure. Correct diagnosis, however, requires proper assessment of multiple hemodynamic parameters by echocardiography, each of which can be influenced by vascular tone, cardiac output, arrhythmia, and concomitant valve lesions. Herein, we review the causes of aortic stenosis, how to assess and classify stenosis severity, and special considerations such as low flow states that should prompt additional workup.