The cardiovascular system can be characterized as a series of chambers connected by tubes and orifices. The circulatory physiology of this system is governed by hydrodynamic laws. The first application of hydrodynamics to stenotic valve orifices was by Gorlin and Gerlin in 1951, with direct measurement of transvalvular pressure gradients in the catheterization laboratory. The relative imprecision of fluid-filled catheters was corrected by the introduction of high fidelity micromanometric catheters in 1978. Echocardiography, which directly measures blood velocity, currently provides an accurate and widely applied tool for hemodynamic evaluation. Measured changes in blood velocity can derive pressure gradients previously measured by cardiac catheterization. In the clinically important range of determinations, there is excellent correlation between echocardiographic methods and the Gorlin formula for calculating valvular stenosis. Although noninvasive evaluation of heart valve stenosis has become standard, the same physical laws apply as in the 1950s, and practitioners need to be aware of the limitations of the various methods of hemodynamic calculation.