A Global Positioning System (GPS) network was established in 1987 in order to measure crustal deformation across the Yellowstone Plateau (centered on the Yellowstone caldera) and adjacent Teton, Wyoming, and Hebgen Lake, Montana, fault zones. Forty-four GPS stations in the Yellow-stone network were re-surveyed in 1989 and station coordinates using broadcast orbits were obtained. These results were used to assess the precision and accuracy of the GPS surveys and to identify areas of rapid crustal deformation on the Yellowstone Plateau. The GPS measured displacements between 1987 and 1989 were calculated relative to a station at the eastern edge of the 100 km-wide network. The relative horizontal displacements have an average magnitude of 15±10 mm. The relative vertical displacements are generally less than 25 mm about an average near zero and with an uncertainty of ±20 mm. Comparisons with leveling and trilateration measurements, taken nearly simultaneously, suggest that the accuracy of these GPS measurements is at the level of the above uncertainties. GPS-measured vertical displacements across the northern part of the Yellowstone caldera and 1st-order leveling results show up to 20 mm of subsidence between 1987 and 1989. This subsidence is part of a six year period of caldera deflation that began in 1985. In addition, up to 66 mm of uplift is observed with GPS at the northwest edge of the Yellowstone caldera and may be associated with processes following an intense earthquake swarm in 1985–86 and with caldera subsidence.