Observations of the global distribution and seasonal variation of the Martian atmospheric water vapor have been made from the Viking orbiters for a continuous period covering a complete Martian year. The seasonal dependence of the latitude distribution of the column abundance of vapor is consistent with a model in which the vapor is in equilibrium with the regolith at polar and mid-latitudes. The results are consistent with there being a permanent reservoir of water ice buried at a depth of 10 cm to 1 m at all latitudes poleward of 40¿. The behavior of the vapor in the summer hemisphere suggests an annual net transport of the vapor phase from the southern to the northern hemisphere, with deposition of ice of thickness of the order of a few milligrams per square centimeter in the northern polar latitudes. The hemispheric asymmetry is the result of the propagation of the global dust storms originating in the south.