Over the past two decades, the value of Global Positioning System (GPS) measurements to solid Earth science, atmospheric science, ocean science, space physics, and other areas has been firmly established in some cases, such as global and regional surface deformation. In other cases, the value of GPS measurements is now emerging or showing a potential. Some researchers believe the time now has come to assess, from a multi-program and multi-discipline perspective, the benefits of a continuing investment in GPS technology development, and to determine its role in the vision and priorities of national research organizations. In conjunction with the 2004 AGU Fall Meeting special focus session on Emerging science applications of measurements from GPS/GNSS and GPS-like signalsc Recent results and future possibilities, a workshop was held on the prior evening, 16 December. Chaired by Jim Anderson of Harvard University, the workshop objective stated in the invitation to participants was ...to identify and articulate the key scientific questions that are optimally, or perhaps uniquely, addressed by GPS (or more generally, the Global Navigation Satellite System, GNSS, to include other constellations such as Galileo) or GPS-like observations, and determine their relevance to existing or planned national Earth-science research programs.