Opportunities to use the Global Positioning System (GPS) for ionospheric total electron content (TEC) research are reviewed. The era of TEC measurements using very high frequency geosynchronous beacons is essentially over, and the new GPS TECs need to be treated in special ways if they are to augment the existing database of total electron content. Data taken at Boulder, Colorado, show that with appropriate smoothing, GPS TECs can be used to extend the existing database. The time delay data from the International GPS Geodynamics Service over central Europe are used to map the total electron content and reveal regional structures. Global electron-density profiles can now be measured using the GPS/MET (meteorology) system. The measurement of precipitable water vapor in the troposphere to an accuracy of 10% requires that 99.9% of the ionospheric delay be removed.¿ 1997 American Geophysical Union