The goal of developing reliable commercial Doppler systems for measuring vessel diameter and velocity changes during the cardiac cycle appears to be near. Reaching this goal would enable us to obtain volume-flow information continuously. In animal experiments, continuous measurements of Doppler velocity, pressure, and flow add important insights into hemodynamic measurements. Incorporating Doppler methods in microcirculatory research could also provide a link between the microcirculatory and the macrocirculatory hemodynamic research. Although Doppler methods have been validated, Doppler findings in clinical research (using commercial systems) must be considered at best to reflect qualitative circulatory alterations indicating directions of change. Because of inherent technologic limitations and considerable intersubject and intrasubject variability, direct extrapolation of the numeric findings from one study to the other can lead to misleading conclusions. The Doppler results are also influenced by measurement conditions and equipment settings. However, Doppler-derived information can be used as an adjunct to clinical management in many of the diseases discussed above. As with any physiologic variable, serial measurements probably are of greater value than single measurements. With continued improvement in technology, Doppler methods hold promise of becoming an important adjunct in cerebral hemodynamic monitoring in perinatal-neonatal intensive care units.