The special issue is motivated by tremendous changes and opportunities in an integrated planning, operation, and modelling of the energy systems, including electric, natural gas, and district heating-cooling systems together with the demand side. The changes are motivated by recent hardware development, integration of renewables and active consumer participation in the energy markets. Widely available and flexible modern hardware enables appetite for coordinated use of the energy technologies. Massive penetration of wind and solar, along with active participation of consumers in energy management, are two other major enablers but also challenges to reliable and profitable operations. Major challenge is due to unprecedented (for the past reality of the 20th century) level of fluctuations and uncertainty the integrated systems need to deal with. The challenges extend across multiple spatial (household to the country size) and temporal (sub-seconds to years) scales. The special issue comprises papers on planning, design, and operation of multi-energy systems as well as the interaction between different energy-carriers.