The multi-partner GEMS project is well-advanced towards its goal of developing a validated, comprehensive analysis and forecasting capability for greenhouse gases, reactive gases and aerosols. The GEMS system assimilates remotely sensed and in-situ weather and composition data to create global distributions of key atmospheric trace constituents in the troposphere and stratosphere at spatial and temporal resolutions similar to those of climate reanalyses. The system is being employed to produce near-real-time and retrospective coupled analyses of atmospheric chemistry, dynamics and thermodynamics; ancillary systems infer surface fluxes of carbon dioxide and methane. GEMS also provides higher-resolution regional forecasts: the global forecasts provide key information on long-range transport of atmospheric pollutants to ten regional air-quality models run cooperatively over a common European domain. An overview of the project will be presented and examples of how the system performs will be discussed. GEMS is funded until mid-2009 by the European Commission; prospects for subsequent routine operation will be outlined.