The GEOS–Chem model is a global 3-D model of atmospheric composition driven by assimilated meteorological observations from the Goddard Earth Observing System (GEOS) of the NASA Global Modeling and Assimilation Office. It is applied by research groups around the world to a wide range of atmospheric composition problems, Central management and support of the model is provided by the Atmospheric Chemistry Modeling Group at Harvard University. The GEOS–Chem community welcomes new members interested in working with the model. GEOS–Chem is a grass-roots cooperative model owned by its user community, and ownership means some responsibilities as listed in the GEOS–Chem management charter. Please contact Model Scientist Daniel J. Jacob to join the user community and access the code.