Ground-based passive radiometer measurements are used to validate satellite-derived cirrus optical depths over the Atmospheric Radiation Measurement Program Southern Great Plains site during March 2000. Optical depths derived from direct beam measurements by a multifilter rotating shadow band radiometer were well correlated with those determined from the Geostationary Operational Environmental Satellite, especially in relatively homogenous cloud fields. Compared to the multifilter rotating shadow band radiometer (MFRSR) results, on average, the satellite retrieval overestimated optical depth by ~0.67 (29%), even though 75% of the GOES values were within ¿1.0 of the MFRSR results. Some of the bias is attributable to cloud inhomogeneities, mismatches in observed clouds, errors in the surface albedo, and possible errors in the ice crystal scattering phase function. The results demonstrate the potential for using MFRSR data, available over many parts of the globe, for validating satellite cloud retrievals in many different surface and atmospheric conditions.