The Mars Exploration Rover Spirit and its Atliena science payload liave been used to investigate a landing site in Gusev crater. Gusev is hypothesized to be the site of a former lake, but no clear evidence for lacustrine sedimentation has been found to date. Instead, the dominant lithology is basalt, and the dominant geologic processes are impact events and eolian transport. I^jany rocks exhibit coatings and other characteristics that may be evidence for minor aqueous alteration. Any lacustrine sediments that may exist at this location within Gusev apparently have been buried by lavas that have undergone subsequent impact disruption.Spirit landed in Gusev crater on 4 January 2004 UTC. It was followed 21 days later by Opportunity, which landed on Meridiani Planurn. Both vehicles landed using a variant of the airbag landing system that was developed for the Mars Pathfinder mission, deploying the rovers after the landers had come to rest on the surface (1). The primary scientific objective of their mission is to explore two sites on the martian surface where water may once have been present, and to assess past environmental conditions at those sites and their suitability for life. Here we provide an overview of the results from the 90-sol (2) nominal mission of Spirit.Like Opportunity, Spirit carries a copy of the Athena science payload (3) (Fig. 1). The topography, morphology, and mineralogy of the scene around the rover have been revealed by two remote sensing instruments: a panoramic camera (Pancam) and a miniature thermal emission spectrometer (Mini-TES). Pancam (4) is a stereo camera whose filters provide 11 unique color spectral bandpasses over the spectral region from 0.4 to 1.1 mm, as well as two other filters for direct imaging of the Sun. Mini-TES (5) produces high spectral resolution (10 cm^') infrared image cubes with a wavelength range of 5 to 29 jjim. The Pancam cameras and the Mini-TES scanning mirrors are mounted atop a mast (3) at a height of ~ 1.5 m above the ground.Once potential science targets have been identified using Pancam and Mini-TES, they have been studied in more detail with the use of two in situ compositional sensors mounted on a 5-degree-of-freedom robotic arm (3). These are an Alpha Particle-X-Ray Spectrometer [APXS (6)] and a Mössbauer Spectrometer (7). Radioactive ^^^Cm alpha sources and two detection modes (alpha and x-ray) in the APXS reveal elemental abundances of rocks and soils (8). The Mössbauer Spectrometer measures the resonant absorption of gamma rays produced by a ^^Co source to determine splitting of nuclear energy levels in ^^Fe atoms, revealing the mineralogy and oxidation state of Fe-bearing phases. The instrument arm also carries a Microscopic Imager [MI (P)] that has been used to obtain highresolution (30 jjim/pixel) images of rock and soil surfaces and a Rock Abrasion Tool [ RAT (10)] that can remove up to ~5 mm of material over a circular area 45 mm in diameter. Finally, the payload includes the Magnetic Properties Experiment consisting of seven magnets that hav...