This chapter provides detail description of aortic regurgitation (AR). Echocardiography is the cornerstone of assessing patients with aortic regurgitation (AR). It allows etiology, mechanism, and severity of AR to be determined as well as the secondary effects on left ventricular (LV) size and function. Serial examinations allow monitoring for disease progression. Echocardiographic information together with patient history and physical examination is the basis on which prognosis is determined and decisions regarding valve surgery are made. AR etiologies, quantification of AR severity, timing of aortic valve surgery, diseases of the aortic valve cusps, diseases of the aortic root, and aortic annulus, M-mode and 2D findings, color Doppler methods, pulsed wave Doppler methods, continuous wave Doppler methods, left ventricle adaptation to aortic regurgitation, mitral valve findings with severe AR are described in detail. The chapter has 18 well illustrated diagrams and images. Two tables are included to highlight etiologies of aortic regurgitation and parameters used to grade AR severity. 36 references are included at the end of the chapter to facilitate further reading.