A definition of the Gulf Stream boundaries is presented together with its most widely used approximations. Positions of the Gulf Stream boundaries, as obtained by radar altimetry, are compared with those determined by infrared and in situ data such as airborne bathythermographs and airborne radiation thermometers. The data used are derived from the Geos 3 satellite, the Experimental Ocean Frontal Analysis, and other experiments conducted by the U.S. Navy Naval Oceanographic Office during February 1976 and January 1977. Results point out that the radar altimeter can sense the Gulf Stream boundaries with as much precision as the infrared-derived data analysis. Both techniques provide excellent agreement with the in situ data. The two sensor systems complement each other, since one is an all-weather profiler and the other a synoptic two-dimensional sensor.