A detailed planetwide stratigraphy for Mars has been developed from global mapping based on Viking images and crater counting of geologic units. The original Noachian, Hesperian, and Amazonian Systems are divided into eight series corresponding to stratigraphic referents. Characteristic crater densities and material referents of each series are (1) Lower Noachian [N(16)] (number of craters >16 km in diameter per 106 km2)>200] basement material; (2) Middle Noachian [N(16)=100-200] cratered terrain material; (3) Upper Noachian [N(16)=25-100 N(5)=200-400] intercrater plains material; (4) Lower Hesperian [N(5)=125-200] ridged plains materials; (5) Upper Hesperian [N(5)=67-125 N(2)=400-750] complex plains material; (6) Lower Amazonian [N(2)=150-400] smooth plains material in southern Acidalia Planitia; (7) Middle Amazonian [N(2)=40-150] lava flows in Amazonis Planitia; and (8) Upper Amazonian [N(2)<40] flood-plain material in southern Elysium Planitia. Correlations between various crater size-frequency distributions of highland materials on the moon and Mars suggest that rocks of the Middle Noachian Series are about 3.92-3.85 b.y. old. Stratigraphic ages compiled for units and features of various origins show that volcanism, tectonism, and meteorite bombardment have generally decreased through Mars' geologic history. In recent time, surficial processes have dominated the formation and modification of rock units. The overall stratigraphy of Mars is complex, however, because of temporal and spatial variations in geologic activity.