The poles and mid-latitudes of Mars contain abundant water in ice caps, thick sequences of ice-rich layers, and mantles of snow. The volume of the known reservoir is ≥5 x 106 km3, corresponding to a layer ∼35 m thick over the planet. Hydrogen in subsurface H2O ice has been detected at latitudes poleward of 50°. Morphological features show downslope flow of ice-rich sediment, and recent gullies have been produced from subsurface aquifers or melting snowpacks. Variations in Mars' orbit on timescales of 50,000 to 2,000,000 years produce significant changes in climate, which result in the transport of water from the poles, where it currently resides, to the lower latitudes, where it may play a critical role in surface geology, mineralogy, and geochemistry.