Presented at Jisc and CNI leaders conference 2018, 2 July 2018, Oxford, UK (https://www.jisc.ac.uk/events/jisc-and-cni-leaders-conference-02-jul-2018). The European Open Science Cloud. What exactly is it? In principle it is conceived as a virtual environment with open and seamless services for storage, management, analysis and re-use of research data, across borders and scientific disciplines. How? By federating existing scientific data infrastructures, currently dispersed across disciplines and Member States. In practice, what it is depends on the stakeholder. To European Research Infrastructures it’s a coordinated mission to organise and exchange their data, metadata, software and services to be FAIR – Findable, Accessible, Interoperable, Reusable – and to use e-Infrastructures, either EU or commercial. To EU e-Infrastructures offering data storage and cloud services, it’s a funding mission to integrate their services, policies and organisational structures, and to be used by the Research Infrastructures. To agencies it’s a means to promote Open Science, standardisation, cross-disciplinary research and coordinated investment with a dream of a “one stop shop” for researchers. And for Libraries?