The X-FLEX project proposes a set of integrated solutions that will facilitate the optimum combination of decentralised flexibility assets, of both on the generation (DER) and on the demand side (V2G, power-to-heat/cold, batteries, demand response) enabling all parties, including final prosumers, to offer their flexibility in the local and wholesale market creating benefits to all the actors in the smart grid value chain. This flexibility will make it possible to maintain a stable and secure electricity system with a growing role of variable renewable generation, yet resilient to extreme climate events. Up to now, the projects and research activities in this area have been focused mainly on specific energy sources and actors, considering only a very limited and isolated part of the distribution network, leaving aside all the opportunities existing throughout the flexibility offered by the energy value chain. In this context, X-FLEX will propose a new concept that will integrate and create synergies among all energy flexibility sources and technologies, promoting cooperation of all the actors of the smart grid and energy market, in an efficient and cost-effective manner. Through this holistic approach, X-FLEX aims to create the optimal combination of decentralised flexibility assets located along the whole energy value chain and new market mechanisms, providing benefits to all the actors of the smart grid and energy market, offering an all-win scenario. These X-FLEX project solutions will be tested in real conditions in 4 pilot sites in 3 EU Member states (Bulgaria, Slovenia and Greece), with different needs and socioeconomic and technological boundaries, involving multiple flexibility assets (batteries, power to heat/cold, vehicle to grid and other storage solutions) and all complementary actors of the energy network (DSO, TSO, microgrid operator, utilities, flexibility providers, local communities).