Integrated energy systems (e.g. integrated electricity/gas/heating/cooling/hydrogen/electrified transport systems) have been developing very fast in Europe

along with the fast development and wide applications of distributed energy resources

technologies of monitoring

control and management and new energy trading in energy systems. This has triggered a further radical revolution of European energy systems and become a new focus of strategic competition and cooperation. The characteristics of integrated energy systems are discussed

drivers and the state of the art (covering both industrial and academic developments) of integrated energy systems are analyzed. It is indicated by the research and practice in Europe that the coupling and interactions among various energy systems are significantly increasing which need a better understanding and optimization. It is a key to improve the performance of the whole energy system and reduce the energy prices. Optimized and controllable integrated energy systems are the physical basis to realize the peer to peer energy trading and promote the development of Energy Internet. This work was partially supported by Research Councils UK and National Natural Science Foundation of China (No. EP/K006274/1

No. 51261130473) and Open Foundation of the State Key Laboratory Program of Tsinghua University (No. SKLD15KZ02).