Geological mapping of Elysium Planitia has led to the recognition of five major surface units, in addition to the three volcanic constructs Elysium Mons, Hecates Tholus, and Albor Tholus. These units are interpreted to be both volcanic and sedimentary or erosional in origin. The volcano Elysium Mons is seen to have dominated constructional activity within the whole region, erupting lava flows which extend up to 600km from the summit. A major vent system, covering an area in excess of 75 000 km², is identified within the Elysium Fossae area. Forty-one sinuous channels are visible within Elysium Planitia; these channels are thought to be analogous to lunar sinuous rilles and their formation in this region of Mars is attributed to unusually high regional topographic slopes (up to ~ 1.7‡). Numerous circumferential graben are centered upon Elysium Mons. These graben, located at radial distances of 175, 205-225, and 330km from the summit, evidently post-dated the emplacement of the Elysium Mons lava flows but pre-dated the eruption of extensive flood lavas to the west of the volcano. A great diversity of channel types is observed within Elysium Fossae. The occurrences of streamlined islands and multiple floor-levels within some channels suggests a fluvial origin. Conversely, the sinuosity and enlarged source craters of other channels suggests a volcanic origin. Impact crater morphology, the occurrence of chaotic terrain, probable pyroclastic deposits upon Hecates Tholus and fluvial channels all suggest extensive volcano-ground ice interactions within this area.