The Mars Exploration Rovers Spirit and Opportunity have both found evidence for water-related processes at their landing sites. In Gusev crater, Spirit found only trace evidence for the action of water on the lava-covered plains of the crater floor. This water action left thin salt-rich deposits on the surfaces of rocks and in fractures within rocks, and what appear to be magnesium sulfate salt concentrations in soils. The older rocks in the Columbia Hills, however, show evidence for substantial alteration by water. At Meridiani Planum, Opportunity has found layered sedimentary rocks that are a mixture of fine-grained silicate particles and chemical sediments precipitated from liquid water. Environmental conditions that these rocks record include episodic inundation by shallow surface water, evaporation, and desiccation. After deposition, the rocks underwent a complex history of modification by water that resulted in recrystallization and growth of hematite-rich concretions. A stratigraphic section obtained within Endurance crater shows significant variation of rock chemistry and texture as a function of depth. Environmental conditions at Meridiani may have been suitable for supporting life at some point in the planet's history.