In the last century, Europe has experienced a growth in its population, economy and urban area. Recent decades also witnessed a rapid rise in global air temperature, attributable to anthropogenic activities (IPCC, 2023). These evolving conditions have significantly changed flows in European streams and rivers, leading to multiple challenges for hydrological sciences, related, for example, to long term variability, climate change, extremes or human alterations of the water cycle. To understand the impacts of these changes, hydrologists need consistent, reliable and long hydrological series. Observations, despite continuous improvements, are still lacking at high enough spatial density across Europe and are often uncertain and discontinuous. One option to overcome these limitations is to rely on a suit of models (climate, hydrological, land use) to simulate past hydrological conditions and interpret changing dynamics in the hydrological cycle in connection with rapidly changing human systems. This work brings together improvements from diverse fields (i.e., remote sensing, climate modelling, machine learning, hydrology) to generate a state-of-the-art hydrological reanalysis: the Hydrological European ReAnalysis (HERA) with a regional coverage of EU27 countries, UK, Switzerland, Iceland, Norway and the Balkan countries (Serbia, Montenegro, Bosnia-Herzegovina, Kosovo, North Macedonia and Albania) over the past 71 years.