We present an efficient and accurate algorithm for the retrieval of water vapor (H₂O) volume mixing ratio (vmr) in the middle atmosphere from satellite measurements of 6.8 μm Earth limb emission, which rigorously includes non-local thermodynamic equilibrium (non-LTE) processes. A number of retrieval studies have been conducted with this algorithm leading to three major conclusions about the retrieval of H₂O in the middle atmosphere: (1) satellite measurements of 6.3 μm limb emission must be made to high altitudes (85–90 km), where the few optically thick H₂O absorption lines become optically thin, in order to prevent large errors in retrieved stratospheric and mesospheric H₂O vmr; (2) the impact of non-LTE on nighttime H₂O retrievals is greater than previously thought; (3) the enhancements in the daytime H₂O channel radiances of the NIMBUS 7 LIMS experiment may be entirely due to non-LTE emission from the H₂O 6.3 μm bands.