Ultrasonic Doppler flowmetry should assume an increasingly important role in the serial evaluation of human cardiovascular physiology. The principal advantages of this method are that it (1) is noninvasive, (2) is nontraumatic, (3) provides anatomic and physiologic data, and (4) provides dynamic measurements. Nevertheless, the equally important limitations of the technique are that it (1) is difficult to apply in some subjects (ie, those who are obese or have anatomic variations), (2) requires operator skill and a thorough knowledge of human anatomy and cardiovascular dynamics, (3) has a finite spatial resolution that may compromise the measurement of small (1 mm) vessel diameters, and (4) has a finite velocity-measuring capability that will affect measurements of blood velocities outside the range of approximately 2 to 200 cm/sec. Full appreciation of the capabilities and limitations of noninvasive ultrasonic Doppler flowmetry makes possible a better understanding of the dynamic interplay of anatomy, pressure, flow, and resistance in the normal and abnormal intact human cardiovascular system.