ELEXIA will develop/upgrade validated tools for planning and managing integrated energy systems in different conditions and will integrate and combine energy systems across vectors and sectors towards a cost-optimised as well as flexible and resilient energy system of systems. A Digital Services Platform will host the energy management and planning services and will foster flexibility and sector coupling. A System Planning Toolbox will be developed and deployed to support effective sector coupling at local sites considering different scenarios, operational details, possibly conflicting interests of multiple local actors, and security of supply. An Energy Management Systems will be built and deployed for flexible, cost-optimised, and resilient operation of sector coupled local sites including forecasting, digital twins, optimization, control, monitoring, assessing operating conditions, predicting anomalous operation, and preventing occurrence of breakdowns. ELEXIA will demonstrate the use of planning and operational tools in a one-stop-shop, modular and open, digital platform at TRL7–8. It will demonstrate the benefits of sector integration at local / national level in three different geographical, climate and economic conditions in Europe: in an industrial port environment in Portugal, in an urban-city hub environment in Denmark, and in an industrial-urban-residential environment in Norway. ELEXIA will assess environmental, economic and social sustainability, will deliver a methodology for CAPEX / OPEX and value creation, and will focus on policy and governance. It will put focus on stakeholder engagement and societal acceptance and will ensure effort towards future exploitation and replication. ELEXIA will establish and demonstrate realistic and concrete pathways to ultimately achieve independence of fossil fuels by harnessing the latent flexibility of the energy system through integration, data-intelligence, and planning, working towards the 2050 European goals.