Due to increasing environmental concerns, there is growing interest amongst researchers, policy makers, and the public in general in better options to make energy more sustainable and, at the same time, ensuring that energy systems are affordable, reliable and resilient. This is bringing about different grand challenges across the world as established energy systems (e.g., in cities) must be enhanced to integrate large volumes of Renewable Energy Sources (RES), while new or evolving systems

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... ., in developing economies) must be planned to cope with increasingly extreme conditions associated with climate change. In these contexts, the flexibility to intelligently use and invest in resources that go beyond the power system (e.g., other energy vectors such as heat or gas and assets such as water dams) can be extremely valuable from a sustainable development perspective. In cities, energy decarbonisation and sustainable development are encouraging the electrification of transports, heating and other services, as well as the large scale integration of RES. Taking the UK as an example, with the aim of decarbonising transports by 2040, all sales of new petrol and diesel cars will be banned by 2032. Also, as heating corresponds to 40% of domestic energy demand in the UK, the government offers a 7 year domestic renewable heat incentive for customers who install Electric Heat Pumps (EHPs) or other forms of renewable heating. At first glance, these solutions seem highly attractive because electricity can be easily decarbonised if it is produced with RES which are becoming progressively cheaper. However, accommodating the new demand and RES generation in the electricity system is not an easy task. Massive investments would be required in electricity grid infrastructure (e.g., lines and substations) to accommodate the new power flows, as well as in generation, storage and other technologies that can provide reserve and active control to balance the highly intermittent output of some RES such as wind and solar Photovoltaic (PV). A more effective approach would be to take advantage of the already existing assets (including district heating and gas networks) and ongoing advances in information and communication technologies (ICT) and automation to enable demand side flexibility, much of which is enabled by multi--energy technologies. This multi--vector approach to demand side flexibility empowers customers to use combinations of energy vectors (e.g., electricity, heat and gas) to better meet their energy needs, while also providing valuable capacity and reserve support to the energy system. The multi--vector vision of energy flexibility recognises the attractiveness of using a suite of energy vectors and networks to meet customer needs. Taking this vision a step further, it may not make sense to constrain flexibility to the energy sector in areas where little or no energy infrastructure has been installed, such as in rural areas or developing economies. Instead, it is more valuable and sensible to consider the flexibility that investments in some infrastructure can offer to different sectors, such as hydropower plants which couple the energy and water sectors and allow flexibility to be deployed to the benefit of other sectors (e.g., releasing water from the energy sector to be used in the agricultural sector). The flexible use of different resources provides new opportunities to more efficiently bring lighting, water, food and other valuable services to underserved customers. However, in the so--called water--energy nexus, this increased flexibility needs to be properly