An increasing number of Earth-looking imaging spectrometers are being developed to derive properties of the surface from measured upwelling spectral radiance that has passed through the atmosphere. However, the measurements are influenced by the amount of atmospheric water vapor. Water vapor is present in every spectrum and is highly variable through space and time. This research assesses the sensitivity to the accurate knowledge of atmospheric water vapor of imaging spectrometers with surface measurement objectives. This sensitivity is explored for a range of imaging spectrometer spectral resolutions from 5 to 20 nm as well as for a range of atmospheric water vapor from normal to dry conditions. The effect of atmospheric water vapor errors of 50, 25, 10, 5, and 1% is analyzed. In all cases the derived surface reflectance is shown to be strongly sensitive to the accurate knowledge of water vapor in the imaging spectrometer measurement. This result leads to a general requirement of 1% or better knowledge of atmospheric water vapor in order to derive surface reflectance that is largely free of water-vapor-induced errors, distortions, and spectral artifacts. ¿ 2001 American Geophysical Union