The Marshall Space Flight Center (MSFC) has been given the responsibility for conceptual development of the Geostationary Earth Observatory (GEO) element of NASA's Mission to Planet Earth program. Because these multi-instrument geostationary satellites will orbit over given points on the ground, they will each provide continuous observation of large regions of the Earth and will complement other data gathering facilities in low Earth orbit (LEO) such as the polar platforms of the Earth Observing System (EOS) and the Earth Probe satellites which operate in a variety of specialized LEO's. These various systems will operate over a 15-year period to obtain data with unprecedented global and temporal coverage. Because of their Earth-fixed position, the GEO instruments will provide high temporal resolution while the LEO instruments will provide data having higher spatial and spectral resolution. These data will be used to define the hydrologic, biogeochemical, and energy cycles which constitute the Earth system. Understanding these cycles and developing the capability to predict their course are the ultimate goals of Mission to Planet Earth.In response to the recommendations of the GEO Science Steering Committee, several key instruments have been provisionally identified to provide the required scientific observations. Some of these instruments were chosen to parallel certain instruments planned for the EOS polar platforms allowing direct intercomparisons. Other instruments, including possibly a set of “operational” sensors and certain dedicated Principal Investigator (PI) sensors, will also be included on GEO.Currently, MSFC is conducting in-house and contracted design studies of the GEO spacecraft based on accommodating the provisionally selected instruments. Stringent instrument pointing and stability criteria are major observatory design drivers. The observatories are baselined for launch on the Titan IV/Centaur but would be a candidate for the National Launch System (NLS) should it become available in the required timeframe.Development of a data and information system which is responsive to the needs of the Earth Science community is a critical part of the early design definition studies now underway. As a goal, data from all GEO's would be available in near-real time to the research community of government, university, and private users. A variety of concepts is under investigation to achieve this goal including satellite linking and ground system linking.It is anticipated that GEO will evolve into a multinational program. One concept calls for five GEO's employed simultaneously around the Earth, three by the United States and two by nations such as Japan or the European Space Agency (ESA). Current technology and engineering studies indicate that the first GEO can be ready for launch shortly after the year 2000.This paper discusses in more detail the scientific rationale, required instrumentation, observatory configuration, and data system of the GEO program.