The two Mars exploration rovers, Spirit and Opportunity, touched down on Mars in January 2004 and have been conducting extensive observations with the Athena science payload. Together the two rovers have traversed ~16 km. Spirit, located on the floor of Gusev crater, has investigated basaltic plains, as well as older materials in the Columbia Hills. The rocks of the Columbia Hills are largely clastic in nature and range from breccias to finely laminated deposits that have undergone significant aqueous alteration. They appear to be largely a mixture of altered impact ejecta and explosive volcanic materials. Recently, Spirit has discovered silica-rich deposits that may have formed in a hot spring or fumarole environment. Opportunity has carried out the first outcrop-scale investigation of ancient sedimentary rocks on Mars. The rocks are sandstones formed by wind and water erosion and re-deposition of fine grained siliciclastics and sulfate-rich evaporites. The stratigraphic section observed to date is dominated by eolian bedforms, with subaqueous current ripples exposed locally near the top of the section. While liquid water was present at Meridiani below and occasionally at the surface, the ancient environmental conditions recorded there are dominantly arid, acidic and oxidizing, and would have posed some significant challenges to life.