The data recorded by a synthetic aperture radar (SAR) can in principle be analyzed to obtain the radial velocity of moving objects in the scene relative to the flight path of the SAR platform. The application of this concept for measuring ocean surface currents has been evaluated using data from SAR systems mounted in aircraft and spacecraft platforms. This paper summerizes the results of these experiments and describes the system effects which can influence the measurement accuracy. Considerations involved in the design of optimum sytems for this application are also discussed.