The Orbiting Carbon Observatory (OCO) mission will make the first global, space-based measurements of atmospheric carbon dioxide (CO 2 ) with the precision, resolution, and coverage needed to characterize CO 2 sources 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 O 2 A-band and in the CO 2 bands at 1.61 and 2.06 μm. Soundings recorded in these three bands will be used to retrieve the column-averaged CO 2 dry air mole fraction (X CO2 ). A comprehensive validation program was included in the mission to ensure that the space-based X CO2 measurements 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 CO 2 sources and sinks. While OCO will have a nominal lifetime of only 2 years, it will serve as a pathfinder for future long-term CO 2 monitoring missions. © 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.