Abstract
Hydropower reservoirs are key elements in the energy system by the provision of flexible energy services. They are expected to play an even more important role in a fully renewable system with an increasing share of intermittent sources, such as wind and solar power. Besides the production of renewable energy, hydropower with reservoirs are important providers of water-related services such as water for irrigation, supply of drinking water and as means to control droughts and floods. This section describes the challenges and the opportunities in the establishment and operation of hydropower reservoirs.