Over the past three years, data from the Mars Global Surveyor and Mars Odyssey spacecraft have revealed startling evidence that water has been active in shaping portions of the martian surface within the last few million years.  This discovery challenges the prevailing view that Mars’ surface has been continuously cold and dry—much as it is today—for the past three to four billion years.  The new data include evidence for extensive near-surface ice at high latitudes, features related to glacial action, gully erosion and debris flows, ponding of water in temporary lakes, modification of terrain by subsurface ice, and the outburst of megafloods, closely related to volcanic eruptive processes.  The causes for the relatively recent environmental/hydrological changes on Mars may involve volcanic and geothermal processes, and they may also involve factors relating to orbital dynamics.  The evidence for this hydrological activity is of profound importance to the planning of future missions to Mars for planetary exploration, including the search for fossil or extant life, and the eventual human presence on the planet.