Multi-energy systems is a research concept and engineering paradigm focused on the integrated analysis and optimal coordination of interconnected infrastructures managing multiple energy carriers, such as electricity, heat, gas, and transportation fuels. This field investigates the synergistic interactions, conversion pathways (e.g., power-to-X), storage mechanisms, and flexible demand within and between these heterogeneous networks (e.g., microgrids, buildings, ports, virtual power plants), employing sophisticated modeling, control strategies, and often data-driven techniques, including artificial intelligence, for prediction and operational optimization. Its significance lies in enabling enhanced overall system efficiency, reliability, resilience, and environmental sustainability while addressing the complex multi-criteria challenges inherent in modern energy landscapes and facilitating the transition to low-carbon energy systems.