Major clinical uses of the new Doppler color flow mapping technologies involve the imaging of disturbed flow through cardiac defects or valves. Nevertheless, there is little general understanding of the determinants of flow and of how flow is imaged by these new systems. This review will attempt to relate the hydrodynamics through a simplified stenotic or regurgitant orifice with the physics and sampling theories relevant to the functioning of Doppler color flow mapping systems. The goal will be to characterize the velocity resolution, spatial resolution, sensitivity and performance of these systems so that clinicians can understand why flow looks the way it does on Doppler color studies and which aspects of flow mapping can be expected to become more quantifiable than they are at present.