The results of observations of the spatial and temporal variation of water vapor during the Viking primary mission are reported. The instrument, the Mars atmospheric water detector (Mawd), is a five-channel grating spectrometer operating in the 1.4-μm water vapor bands. The seasonal period covered here is the northern summer solstice to the following equinox. The global water vapor, mapped at low resolution at approximately 1-month intervals, has been observed to undergo a gradual redistribution, the latitude of maximum column abundance moving from the northern polar area to the equatorial latitudes, and the integrated global atmospheric vapor content remaining constant. The peak abundances (∼100 precipitable microns) occurred over the dark material of the circumpolar region. The summer residual cap is dirty water ice; at the season of maximum vapor the atmosphere above it is saturated and has a stable lapse rate, of temperature. High-resolution maps show local structure controlled by abrupt changes of surface elevation, suggesting that large variations at a given latitude are orographie in nature and only occur in association with features whose horizontal scale is small in comparison to the product of the atmospheric relaxation time and the local mean wind speed. These results are at variance with the low-resolution global maps, however, which seem to show topographic control even at the regional scale. Attempts to isolate the diurnal variation of the vapor have shown a variety of effects at different latitudes and locations; scattering by dust and condensate particles obscures the intrinsic diurnal variation of the vapor phase. The large diurnal variation reported from earth-based measurements may be largely an observational effect.