The efficiency and feasibility of geothermal utilisation depends strongly on the characteristics and behaviour of the fluids that transfer heat between the geosphere and the engineered components of a power plant. Chemical and physical processes such as precipitation, corrosion, or degassing are induced by pressure and temperature changes, with potentially serious consequences for power plant operation and project economics. The EU Horizon 2020-funded project REFLECT aims to avoid such problems by collecting high-quality chemical, physical, and microbiological data at extreme salinities, pressures or temperatures and improving the understanding of kinetic processes through laboratory experiments. These data are presented in a European geothermal fluid atlas and implemented in predictive models in order to provide recommendations on how to best operate geothermal systems for a sustainable future.