Renewable hydrogen combined with large scale underground storage enables transportation of energy through time, balancing out the impacts of variable renewable energy production. While storing pure hydrogen in salt caverns has been practiced since the 70s in Europe, it has never been carried out anywhere in depleted fields or aquifers. Technical developments are needed to validate these two solutions. As subsurface technical feasibility studies for a future hydrogen storage in depleted field or aquifer will be site-specific, as for other geology related activities, HyStories will provide developments applicable to a wide range of possible future sites: the addition of H2-storage relevant characteristics in reservoir databases at European scale; reservoir and geochemical modelling for cases representative of European subsurface, and tests of this representativeness by comparing it with results obtained with real storage sites models; and lastly an extensive sampling and microbiological lab experiment programme to cover a variety of possible conditions. Complementarily, techno-economic feasibility studies will provide insights into underground hydrogen storage for decision makers in government and industry. Modelling of the European energy system will first define the demand for hydrogen storage. Environmental and Societal impact studies will be developed. For a given location and hydrogen storage demand, a high-level cost assessment for development of each of the competing geological storage options at that location will be estimated, and the sites will be ranked based on techno-economic criteria developed within the project. Finally, several case studies will enable consideration of the implementation of potential projects, notably by considering their economic interest. This will provide substantial insight into the suitability for implementing such storage across EU and enable the proposition of an implementation plan.