TThe Intergrated Carbon Observation System (ICOS) is a distributed European research infrastructure (RI) producing high-precision data on greenhouse gases. It provides highly standardised and open data from 180 measurement stations across 16 European countries. The stations observe greenhouse gas concentrations in the atmosphere as well as carbon fluxes between the atmosphere, land surface and oceans. Thus, ICOS is rooted in three domains: Atmosphere, Ecosystem and Ocean. Overall, ICOS community consists of more than 500 scientists in Europe and beyond. More than 80 renowned universities or institutes are a part of the ICOS community which has also strong connections to colleagues and operators outside academia. ICOS RI facilitates research to understand the carbon cycle and to provide necessary information on greenhouse gases. ICOS-based knowledge supports policy- and decision-making to combat climate change and its impacts. ICOS is the European pillar of a global greenhouse gas observation system. It promotes technological developments and demonstrations related to greenhouse gases by linking research, education and innovation. Entered the ESFRI Roadmap in 2006, ICOS established the European Research Infrastructure Consortium (ERIC) in 2015. ICOS is fully operational. ICOS data are integrated into major global data systems and networks and meet the needs of international requirements, principles and agreements. ICOS Thematic Centres and Central Laboratories are established as centres of excellence in their domains and provide stable support for European and international networks. The ICOS data are available near-real-time, fully open and compliant with FAIR principles at the ICOS Carbon Portal. Beyond the data provision, the ICOS Carbon Portal provides computational services to scientists to analyse the data and to run models on greenhouse gas fluxes. ICOS represents a strong European community of institutions investing into in situ observations, and it has established itself as an organisation from which relevant stakeholders seek advice. The World Meteorological Organisation (WMO) has identified ICOS as a blueprint for its Global Greenhouse Gas Watch (G3W) initiative. ICOS acts as a mediator between the greenhouse gas science community and the agencies that work on national inventories and the global stocktake. ICOS drives technical developments in order to improve analytical capacities, supporting industry partners to fulfil its high-quality needs. Within the ensemble of ENVRI, ICOS is a strong European building block and supporter of a recently established global RI on ecosystem research. This role, the benefits of ICOS to science and society, and an increased number of member countries have resulted in obtaining stable and sufficient resources to further sustain and develop the RI.