ICOS consists of a network of standardized, long-term, high-precision integrated monitoring of atmospheric greenhouse gas concentrations and fluxes. The infrastructure integrates terrestrial and atmospheric observations at various sites into a single, coherent, highly precise dataset. This data allows a unique regional top-down assessment of fluxes from atmospheric data, and a bottom-up assessment from ecosystem measurements and fossil fuel inventories. Target is a daily mapping of sources and sinks at scales down to about 10 km, as a basis for understanding the exchange processes between the atmosphere, the terrestrial surface and the ocean. ICOS contributes to the implementation of the Integrated Global Carbon Observation System IGCO.