The need for a reliable, low-cost observing system to measure water vapor in the atmosphere is incontrovertible. Experiments have shown the potential for using Global Positioning System (GPS) receivers to measure total precipitable water vapor accurately at different locations and times of year and under all weather conditions. The National Oceanic and Atmospheric Administrations's (NOAA) Forecast Systems Laboratory (FSL) and Environmental Technology Laboratory (ETL), in collaboration with the University NAVSTAR Consortium, University of Hawaii, Scripps Institution of Oceanography, and NOAA's National Geodetic Survey (NGS) Laboratory, are addressing this need by developing a ground-based water vapor observing system based on the measurement of GPS signal delays caused by water vapor in the atmosphere. The NOAA GPS Integrated Precipitable Water Vapor (NOAA GPS-IPW) network currently has 35 continuously operating stations and is expected to expand into a 200-station demonstration network by 2004. This paper describes the major accomplishments of the project since its inception in 1994. Results from the analysis of the effect of satellite orbit accuracies on IPW accuracy are discussed. Several comparisons with collocated remote and in situ measurements, including radiosondes and ground- and space-based radiometers are shown. Results from preliminary model runs using the FSL Forecast Research Division's Mesoscale Analysis and Prediction System (MAPS) model are presented. This work shows the feasibility of an operational system using GPS to continuously monitor atmospheric water vapor in near-real time with accuracies (&lt; 1.5 cm) comparable to radiosondes and water vapor radiometers.