The interdependencies between water and energy are well known and they have become a topic of increasing attention for the scientific and policy communities. Water is used throughout the energy industry, and the water system needs energy for collecting, pumping, treating and desalinising water. Increasing water and energy needs, or changes in water availability due to climate change could have significant effects on the energy system. These problems are expected to be very acute in developing countries, but also in Europe. The EU has ambitious decarbonisation goals for the future, which could be very difficult to achieve if the European water system becomes too stressed, since decarbonisation relies on water-demanding energy technologies such as biofuels, carbon capture, or nuclear power. The water sector is not as energy-intensive as other industries. Despite that, the operation of the water sector may offer solutions for increasing the flexibility of the European power system. This may be achieved by powering water treatment and desalination plants with renewable energy and by using water supply and distribution networks to store energy. All the above considerations denote that the use and management of energy and water resources need to be addressed simultaneously, especially when taking into consideration that the fundamental difference between energy and water is that energy can be renewable, however water resource are finite. Only with this “nexus” approach it is possible to take full advantage of the opportunities to increase energy efficiency in the water sector; to exploit the possibilities of the water system as a source of flexibility for the power system; to extract more energy from water; and to reduce the water footprint of the energy industries. The Water Energy Food and Ecosystem Nexus (WEFE Nexus) flagship project addresses in an integrated way the interdependencies and interactions between water, energy, agriculture, water supply and treatment, as well as the environment. These interactions have been so far largely underappreciated. The WEFE-Nexus can be depicted as a way to overcome stakeholders’ view of resources as individual assets by developing an understanding of the broader system. It is the realisation that acting from the perspective of individual sectors cannot help tackle future societal challenges. This report summarises the main results obtained to date within the WEFE project as regards the water-energy nexus.