This short paper gives some perspectives on the recent development in the concept of Smart Energy Systems while introducing the contents of selected papers from the 5th International Conference on Smart Energy Systems, 4th Generation District Heating, Electrification, Electrofuels and Energy Efficiency – SESAAU2019. All of the papers elaborate on or contribute to the theoretical scientific understanding on how to design and implement an energy efficient green transition with an emphasis on system integration and sector-coupling. Low-temperature 4th generation district heating and its possibilities of providing flexible demand is particularly emphasized in the papers presented. However, in order to reach 100% renewability in the energy system, further research is required particularly on the transformation of the transport and industrial sectors. Methods for accommodating increased levels of renewable energy sources, while maintaining proper load-balancing capability and security of supply, should also be further investigated – an area where smart energy systems play an important role.