An airborne scanning spectrometer has been developed for measuring reflected solar and emitted thermal radiation in 50 narrowband channels between 0.55 and 14.2µm. The instrument provides multispectral images of outgoing radiation for purposes of developing and validating algorithms for the remote sensing of cloud, aerosol, water vapor and surface properties from space. The spectrometer scans a swath width of 37 km, perpendicular to the aircraft flight track, with a 2.5 mrad instantaneous field of view. Images are thereby produced with a spatial resolution of 50 m at nadir from a nominal aircraft altitude of 20 km. Nineteen of the spectral bands correspond closely to comparable bands on the Moderate Resolution Imaging Spectroradiometer (MODIS), a facility instrument being developed for the Earth Observing System (EOS) to be launched in the late 1990s. This paper describes the optical, mechanical, electrical, and data acquisition system design of the MODIS Airborne Simulator (MAS), and presents some early results obtained from measurements acquired aboard the NASA ER-2 aircraft that illustrate the performance and quality of the data produced by this instrument.