An overview of the use of high-frequency ultrasound to map blood flow through the microvasculature will be presented. The correlation of scattered echoes from blood at frequencies from 20 to 100 MHz are shown to demonstrate a smaller probability of error than the correlation at a lower transducer frequency. Experimental three-dimensional (3-D) maps of the velocity profile within vessels with a diameter of 100 (mu)m or less will be presented. Blood velocity within vessels as small as 40 (mu)m can be mapped using a transducer with a nominal center frequency of 50 MHz. Estimation of flow within these small vessels at the appropriate low velocities is challenging due to the small signal levels, the effect of cardiac and respiratory motion, and the unknown rate of fluctuation of the received signal from the small number of red blood cells contained within these sample volumes. Reliable in vivo estimation of the velocity within the microvasculature requires realignment of the signal from a single line of sight, and this method is presented. Recognition of 3-D continuity of a vessel tree is required to eliminate spurious noise. The rate of fluctuation in the received signal from small blood vessels is evaluated as a function of the sample volume and vessel size. A minimum time window for reliable detection of flow through a small vessel is determined. Finally, the use of acoustic contrast agents to improve the visualization of flow within small vessels will be introduced.