The commitment made under the Paris Agreement to cut greenhouse gas emissions has been an important contributor to the increasing share of renewables in the European energy system. However, the variability of some renewable energy sources such as wind and solar poses a challenge when included in the energy system because it must be balanced by other energy sources. Hydropower, a renewable energy technology with a low carbon footprint, is such a balancing resource. Making hydropower more flexible will thus further increase its value in a future energy system. Therefore, the HydroFlex project focuses on key bottlenecks of hydropower plants that restrict their flexibility today while at the same time addressing the environmental and socio-economic impacts of increased flexibility.