After an intensive 3-month review of several high science value mission candidates during the summer of 2000, the Mars Exploration Rover (MER) mission was officially selected by NASA in August 2000 as an approved flight mission for launch in the 2003 launch window. The decision to select twin MER rovers for launch in 2003 was based in part on arguments related to the reduction of the risks associated with launch failure as well as the unknowns of Martian environmental conditions at the time of landing. The MER mission will significantly enhance our scientific understanding of the local Martian surface by combining a powerful science payload with mobility measured at the scale of several orbital remote-sensing pixels (i.e., hundreds of meters). There is a vast storehouse of orbital remote-sensing data now available to the science community, and these new data sets show that Mars is a more dynamic, perhaps more water-rich, profoundly layered planet in comparison to Viking-based interpretations developed during the period from the mid-1970s to the middle 1990s. MER will provide invaluable ground truth observations with which to place the wealth of orbital remote-sensing data from the ongoing Mars Global Surveyor and Mars Odyssey missions in scientific context and to improve community-based assessments of the validity of orbital-based observations. By sending each rover to a specific landing site that shows evidence of past liquid water activity [Golombek et al., 2003], the MER mission will broaden scientific understanding of the role of water on Mars and directly explore the possibility of biologically hospitable environments having existed at certain locations on Mars. The MER mission employs an integrated suite of synergistic scientific instruments [Squyres et al., 2003] to analyze and understand whether surficial rocks and soils formed or were modified in the presence of liquid water, thereby addressing one of the major thematic objectives of the current Mars Exploration Program.
 The scientific results of the MER mission will exert an important impact on both the present and upcoming decade of Mars exploration (Figure 1) [Garvin et al., 2001]. Immediately following MER, the 2005 Mars Reconnaissance Orbiter (MRO) will be deployed to identify the most scientifically compelling sites where the effects of liquid water or hydrothermal systems are suggested and thereby a suite of high-priority future landing sites relevant to key issues associated with biologic “habitability.” Results from MER will feed directly into the prioritized targeting of the two high-resolution MRO science instruments, Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), a hyperspectral imaging spectrometer, and High Resolution Imaging Science Instrument (HiRISE), a submeter-resolution, multicolor camera, and will serve to provide local validation for the entire suite of MRO observations. The MER experience will influence the design and development of the 2009 Mars Science Laboratory (MSL), which will be sent to the most accessible, high-science-priority site identified on Mars via MRO for the purpose of understanding one specific paleoenvironment (i.e., potentially one associated with preserved layered aqueous sediments), searching for potential biosignatures, and characterizing the building blocks of life, if any are preserved. The MSL mission is currently baselined to make use of a radioisotope power system, with a landing precision about an order of magnitude better than that available to the 2003 MER mission (i.e., <10 km semimajor access of the landing error ellipse, 3 sigma). Because a major objective of MSL is to perform definitive analytical measurements of rocks and soils in order to search for evidence of potential biosignatures, MER represents a stepping-stone toward the challenging goal of understanding how to look for ancient life on Mars that can be directly applied to targeting MSL analytical measurements. By the completion of the MER mission, the new understanding of the surface of Mars provided by these roving laboratories will be used as guidance to select future landing sites for MSL, as well as the types of investigations that must be accomplished as part of the MSL mission.
 This special section of JGR-Planets assembles a collection of papers that describe the Mars Exploration Rover mission, its science payload and specific investigations, current candidate landing sites, and the science activities that could be carried out at those candidate sites. It also includes updated calibration and results from the Pathfinder Alpha Proton X-Ray Spectrometer, which are directly relevant to results to be attained from the MER Alpha Particle X-Ray Spectrometer. Several of the papers in this section are a result of the high level of interest generated at four Mars Exploration Rover landing site workshops that were open to the entire science community [Golombek et al., 2003], which were held 24–25 January 2001, 17–18 October 2001, 26–28 March 2002, and 8–10 January 2003. These workshops provided essential inputs to the process of characterizing and selecting landing sites for the Mars Exploration Rover mission and helped develop community consensus concerning those science hypotheses that could be tested with the rovers and their associated payloads at the final candidate landing sites selected by the NASA Associate Administrator for Space Science.