Mountains are among the most sensitive regions to climate change on Earth. Impacts of climate change are being increasingly observed in mountains, and related future projections raise tremendous concerns of threats to sustainable development. Water is a key source of life for mountain societies and ecosystems, as well as for hundreds of millions of people in downstream areas. However, changing river flow regimes due to rapidly declining glacier ice and snow and changing precipitation patterns jeopardize this ecosystem service over the mid- to long-term, with potentially dramatic impacts for agriculture and energy production. Hydropower generation has furthermore gained increasing importance in the context of the necessary energy transformation to mitigate further anthropogenic driven climate change by exploiting renewable energy potentials. Yet, mountain areas are sensitive social-ecological systems, and the local people are often marginalized and exposed to unequal power relations and competing and conflicting water demands. Achieving sustainable development under these increasing stressors and constraints is a tremendous challenge that can only be addressed collectively.