The roots of GPS sounding go back to the first days of interplanetary flight. In the early 1960's teams from Stanford University and the Jet Propulsion Laboratory exploited radio links between Earth and the Mariner 3 and 4 spacecraft to probe the atmosphere and other properties of Mars. Radio science has since been a staple of planetary exploration. Before the advent of GPS, application of radio occultation to Earth was deemed impractical, and it was not immediately evident that even GPS could be usefully adapted. Techniques devised for geodesy in the 1980's, including "codeless" carrier reconstruction, clock elimination by double differencing, GPS-based precise orbit determination, and cormpact. low-cost receivers, set the stage. JPL submitted the first GPS occultation proposal to NASA in 1988. Though that "GPS Geoscience Instrument" did not fly, it established the concept and led soon to the GPS/MET experiment, conceived by the University Corporation for Atmospheric Research and sponsored by the US National Science Foundation. The remarkable success of GPS/MET has led NASA to mount follow-on experiments on five international missions, launching between 1999 and 2001. Those will refine the systems and techniques of GPS sounding, setting the stage for COSMIC-the first operational GPS occultation constellation.