The Orbiting Carbon Observatory (OCO) mission will make the first global, space-based measurements of atmospheric carbon dioxide (CO2) with the precision, resolution, and coverage needed to characterize CO2sources and sinks on regional scales. The measurement approach and instrument specifications were determined through an analysis of existing carbon cycle data and a series of observing system simulation experiments. During its 2-year mission, OCO will fly in a 1:15 PM sun-synchronous orbit with a 16-day ground-track repeat time, just ahead of the EOS Aqua platform. It will carry a single instrument that incorporates three bore-sighted high-resolution spectrometers designed to measure reflected sunlight in the 0.76-μm O2A-band and in the CO2bands at 1.61 and 2.06 μm. Soundings recorded in these three bands will be used to retrieve the column-averaged CO2dry air mole fraction (XCO2). A comprehensive validation program was included in the mission to ensure that the space-based XCO2measurements have precisions of ∼0.3% (1 ppm) on regional scales. OCO measurements will be used in global synthesis inversion and data assimilation models to quantify CO2sources and sinks. While OCO will have a nominal lifetime of only 2 years, it will serve as a pathfinder for future long-term CO2monitoring missions. © 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.