Cardiac catheterization and Doppler echocardiography are two methods used to measure transvalvular gradients and valve area in the assessment of aortic stenosis severity. Although both approaches are based on the same hemodynamic concepts and report data using the same units of measure, each method measures pressure drop or gradient at a different place; hence they produce fundamentally different quantities. Likewise, cardiac catheterization formulas for valve area attempt to obtain the anatomic area whereas the Doppler continuity equation reports the area to which flow is constricted. To use these two methods appropriately, it is necessary to understand the underlying hemodynamic principles and the effects of the methods of measurement on the values obtained. This article examines these variables and shows how they affect the reported gradients and valve areas and how differences can affect clinical application.