Mars accumulated and differentiated into crust, mantle and core within a few tens of millions of years of Solar System formation. Formation of Hellas, which has been adopted as the base of the Noachian period, is estimated to have occurred around 4.1 to 3.8 Gyr ago, depending on whether or not the planet experienced a late cataclysm. Little is known of the pre-Noachian period except that it was characterized by a magnetic field, subject to numerous large basin-forming impacts, probably including one that formed the global dichotomy. The Noachian period, which ended around 3.7 Gyr ago, was characterized by high rates of cratering, erosion, and valley formation. Most of Tharsis formed and surface conditions were at least episodically such as to cause widespread production of hydrous weathering products such as phyllosilicates. Extensive sulfate deposits accumulated late in the era. Average erosion rates, though high compared with later epochs, fell short of the lowest average terrestrial rates. The record suggests that warm, wet conditions necessary for fluvial activity were met only occasionally, such as might occur if caused by large impacts or volcanic eruptions. At the end of the Noachian, rates of impact, valley formation, weathering, and erosion all dropped precipitously but volcanism continued at a relatively high average rate throughout the Hesperian, resulting in the resurfacing of at least 30% of the planet. Large water floods formed episodically, possibly leaving behind large bodies of water. The canyons formed. The observations suggest the change at the end of the Noachian suppressed most aqueous activity at the surface other than large floods, and resulted in growth of a thick cryosphere. However, presence of discrete sulfate rich deposits, sulfate concentrations in soils, and occasional presence of Hesperian valley networks indicates that water activity did not decline to zero. After the end of the Hesperian around 3 Gyr ago the pace of geologic activity slowed further. The average rate of volcanism during the Amazonian was approximately a factor of ten lower than in the Hesperian and activity was confined largely to Tharsis and Elysium. The main era of water flooding was over, although small floods occurred episodically until geologically recent times. Canyon development was largely restricted to formation of large landslides. Erosion and weathering rates remained extremely low. The most distinctive characteristic of the Amazonian is formation of features that have been attributed to the presence, accumulation, and movement of ice. Included are the polar layered deposits, glacial deposits on volcanoes, ice-rich veneers at high latitudes, and a variety of landforms in the 30–55° latitude belts, including lobate debris aprons, lineated valley fill and concentric crater fill. Most of the gullies on steep slopes also formed late in this era. The rate of formation of the ice-related features and the gullies probably varied as changes in obliquity affected the ice stability relations.