The Mars Global Surveyor spacecraft was placed into Mars orbit on September 11, 1997, and by March 9, 1999, had slowly circularized through aerobraking to a Sun-synchronous, near-polar orbit with an average altitude of 378 km. The science payload includes the Mars Orbiter Camera, Mars Orbiter Laser Altimeter, Thermal Emission Spectrometer, Ultrastable Oscillator (for Radio Science experiments), and Magnetometer/Electron Reflectometer package. In addition, the spacecraft accelerometers and horizon sensors were used to study atmospheric dynamics during aerobraking. Observations are processed to standard products by the instrument teams and released as documented archive volumes on 6-month centers by the Planetary Data System. Significant results have been obtained from observations of the interior, surface, and atmosphere. For example, Mars does not now have an active magnetic field, although strong remanent magnetization features exist in the ancient crust. These results imply that an internal dynamo ceased operation early in geologic time. Altimetry and gravity data indicate that the crust is thickest under the south pole, thinning northward from the cratered terrain to the northern plains. Analysis of altimetry data demonstrates that Mars is egg-shaped with gravitational equipotential contours that show that channel systems in the southern highlands drained to the north, largely to the Chryse trough. A closed contour in the northern plains is consistent with the existence of a great northern ocean. Emission spectra of low-albedo regions show that basaltic rocks dominate spectral signatures on the southern highlands, whereas basaltic andesites dominate the northern lowlands. The bright regions show nondiagnostic spectra, similar to that of dust in the atmosphere. Signatures of aqueous minerals (e.g., clays, carbonates, and sulfates) are noticeably absent from the emission spectra. High spatial resolution images show that the surface has been extensively modified by wind and that layering is nearly ubiquitous, implying that a complex history of events is recorded in surface and near-surface materials. Altimetry data imply that both permanent caps are composed of water ice and dust, with seasonal covers of carbon dioxide frost. Finally, the altimetry data, coupled with thousands of atmospheric profiles, are providing new boundary conditions and dynamic controls for the generation and testing of more realistic dynamic models of the global circulation of the atmosphere. ¿ 2001 American Geophysical Union