Evolution of the continuity equation in the Doppler echocardiographic assessment of the severity of valvular aortic stenosis. The use of Doppler techniques has greatly enhanced the noninvasive ultrasound technique for evaluation of valvular aortic stenosis. M-mode and two-dimensional echocardiography could not reliably distinguish patients with severe aortic stenosis from those with milder obstructions. The hemodynamic information offered by Doppler complemented echocardiographic imaging and provided an alternative modality for evaluation of patients with aortic stenosis. By application of the modified Bernoulli equation, the pressure gradient across the stenotic aortic valve could be estimated by Doppler echocardiography. Though helpful and widely used, the information provided by the pressure gradient across the valve about the severity of the obstruction was not complete. The assessment of valvular aortic stenosis therefore includes an estimation of the valve area by application of the continuity equation. This review examines the maturation of the continuity equation by Doppler techniques and discusses the implications of the procedure. 