Imaging spectrometry from geostationary earth orbit (GEO) can provide the frequently-refreshed detailed information on physical properties of earth's atmosphere and surface needed to enable critical new science missions and ultimately improve operational weather forecasting. We describe and evaluate a concept for imaging spectrometry from GEO that addresses both traditional imaging and sounding applications. Our Geostationary Wedge-filter Imager-Sounder (GWIS) uses spatially variable wedge filter spectrometers to collect earth radiance with approximately 2 km resolution over a 710 - 2900 cm-1 (3.45 - 14.0 micrometer) spectral range at 1% spectral resolution. The resulting instrument, based on LWIR and MWIR wedge spectrometer technology recently developed by Raytheon, is a compact, rugged imager-sounder with better sensitivity, spectral resolution, spatial resolution and full disk coverage time than the current multispectral GOES imager. GWIS sounding performance was simulated by evaluating retrieval performance with respect to a global database of 119,694 cloud-free samples using a stepwise regression algorithm. Retrieved atmospheric parameters included surface air temperature, surface skin temperature, surface water vapor, total precipitable water vapor, total ozone and vertical profiles of temperature and water vapor. In all cases, GWIS outperformed the current GOES sounder. Furthermore, GWIS RMS error performance approached that of advanced higher spectral resolution sounders (e.g., 1.2 K/1 km for GWIS versus 1 K/1 km for advanced sounders). Due to its higher spatial resolution and more complete spatial coverage, GWIS achieves this high quality cloud free sounding performance roughly two times more frequently than high spectral resolution advanced sounders. Combining this new technology with proven wedge spectrometer approaches for visible and near-infrared wavelengths would provide imaging- sounding data from GEO with unprecedented detail and fidelity for a wide range of weather, climate, land use, ocean color and other earth science studies.