Since the advent of meteorological satellites in the 1960's, numerous experiments have been conducted in order to evaluate the impact of these and other data on atmospheric analysis and prediction. Such studies have included both OSE'S and OSSE's. The OSE's were conducted to evaluate the impact of specific observations or classes of observations on analyses and forecasts. Such experiments have been performed for selected types of conventional data and for various satellite data sets as they became available. The OSSE's were conducted to evaluate the potential for future observing systems to improve NWP and to plan for the Global Weather Experiment and more recently for EOS. In addition, OSSE's have been run to evaluate trade-offs in the design of observing systems and observing networks , and to test new methodology for data assimilation . At the present time, an extensive set of OSSEs is being conducted at the NASA Data Assimilation Office in order to determine the requirements for advanced sensors capable of both improving forecasting skill and increasing understanding of the hydrologic cycle. These experiments have shown significant potential for lidar wind profiles to improve analysis and prediction of weather features and also suggest an enhanced capabilty to represent atmospheric processes related to the water cycle. At the conference, advances in OSSE methodology and results from these recent experiments will be presented.