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Hydrothermal systems in deep carbonate bedrock are among the most promising low-enthalpy geothermal plays. Across Europe, apart from a few areas where viability of hydrothermal heat and power generation has been proved, most deep carbonate bedrock has received relatively little attention, because such rocks are perceived as ‘tight’. Exploration and development of the deep subsurface is an acknowledged high-risk investment, particularly in low-enthalpy systems, where tapping suitable temperatures for geothermal energy commonly requires drilling to depths of more than 3 km. In order to de-risk this geothermal exploration it is crucial to improve our understanding of generic geological conditions that determine the distribution and technical recoverability of their potential resources, specifically the possible groundwater yield controlled by fracture conduits and karstification. HotLime is one of 15 projects under the GeoERA umbrella that has received funding from the European Union's Horizon 2020 research and innovation programme. From July 2018 to June 2021 mapping, characterization and comparison of geological situations, the structural inventory of hydrothermal plays in deep carbonate rocks and their petro- and hydro-physical characteristics is carried out in 11 different target areas across Europe in order to identify the generic structural controls of geothermal plays in carbonates. The consistent assessment and the sharing of knowledge among the 15 European partners are geared towards uniformly applicable best practice workflows for estimation, comparison and prospect ranking of these hydrothermal resources. The principal outcomes of HotLime presently prepared will be spatial representations of the areas under investigation (3D models, 2D map series) on the principal geological features and properties relevant for geothermal exploration and production, supplemented by glossaries and a knowledge base including methods and tools which can be transferred and adapted to other carbonate rock suites.