The contribution of Citizen Science (CS) in scientific research has grown exponentially in recent decades, every year millions of data are generated globally through citizen observatories. In fact, citizen science is considered one of the eight priorities of the European Science Agenda, together with the creation of the European Open Science Cloud (EOSC) and the FAIR data initiative. In this context, COS4CLOUD aims to integrate citizen science in the EOSC through the co-design of innovative services to solve challenges faced by citizen observatories, while bringing citizen science projects as a service for the scientific community and the society and providing new data sources. Who are the partners? Research organization: ✔️CSIC ✔️CREAF ✔️INRIA Universities: ✔️The Open University ✔️Swedish University of Agricultural Sciences ✔️National and Kapodistrian University of Athens NGO: European Citizen Science Association Trébola Organización Ecológica SMEs: ✔️Earthwatch ✔️DynAIkon ✔️Bineo ✔️DDQ ✔️Secure Dimensions Non-profit organization: ✔️52N Which are the citizen science projects that participate in Cos4Cloud? Citizen observatories in biodiversity domain: • iSpot • Artportalen • Natusfera • PlantNet Citizen observatories in environmental quality domain: • Fresh Water Watch • Odour Collect • Ispex • kdUINO • CANAIRIO