Interpretation of data from the Mars Global Surveyor Mission (MGS), recent studies of martian meteorites, and new insights into climate modeling have, over the last few years, sharply changed our views about the history of water on Mars. MGS imaging data (MOC) have provided strong (though not unique) evidence of subsurface liquid water within recent times (< 1 Ma) only a few hundred meters from the surface. V-shaped gullies, seemingly formed by running water, emerge from slope faces, have varying degrees of sinuosity, and are often associated with depositional fans. At the ancient (~4 Ga) extreme of Mars’ observed geological history, MOC images reveal layered deposits, in a number of locales, which may be of sedimentary origin and associated with water. The origin of this layered terrain is a fundamental question for understanding the evolution of Mars. Laser altimeter data (MOLA) from MGS have shown that parts of Mars have undergone extensive erosion, in the interval ~3.5-4 Ga, which is most likely fluvial in origin. Several million cubic kilometers of material were removed from the martian highlands in the vicinity of the prime meridian and presumably transported to the northern lowlands. This erosional episode removed previously existing valley networks, which themselves are likely of fluvial origin. Which processes are important (groundwater discharge, surface runoff, sapping) is controversial. Analysis of MOLA topography reveals that much of the valley network formation appears to follow the emplacement of most of the magmatic mass of the Tharsis rise. Volatile release to the atmosphere from Tharsis construction could have significantly perturbed the climate. Recent analyses of martian meteorites indicate that water was relatively abundant in martian magmas, even in the latter part of the planet’s history. Climate modeling involving scattering from CO₂ clouds shows that Tharsis volatile release could have led to the clement conditions potentially required for the creation of valley networks and large-scale fluvial erosion. The recent gullies, the valley networks and large-scale erosion/deposition from ancient times, and the massive water transport inferred from outflow channels formed within the interval ~1-2 Ga, imply that water has been an active geological agent throughout much of martian history.