Earthquake Early Warning (EEW) systems use earthquake science and monitoring technology to alert people when shaking waves generated by an earthquake are expected to arrive at their location. The seconds to minutes of advance warning can allow people to take actions to protect life and property. Research shows, however, the alerts generated by EEW can be improved by incorporating data from the Global Navigation Satellite System (GNSS). At least once per second, GNSS sensors directly measure the 3D displacement of the Earth’s surface during the earthquake, and the EEW processes the data in real time. These observations allow more accurate magnitude estimates, especially for large earthquakes, than seismic data alone and help constrain the estimated length of the fault rupture. Better magnitude and rupture length information in turn enables more accurate predictions of the strength of ground shaking to be expected at any particular location. The USGS Earthquake Science Center is working on several aspects of incorporating GNSS data into EEW. These include establishing more robust real-time GNSS data acquisition and upgrading seismic stations to include GNSS receivers, optimizing the processing strategy required to obtain positions from raw GNSS data in real -time, developing algorithms for inferring earthquake source parameters such as magnitude and length from the GNSS-recorded position streams both independently and jointly with seismic data, and operational implementation of algorithms based on research prototypes. These activities contribute to the overall West Coast EEW effort and to the planned California statewide EEW system.