The transition to a decarbonized energy system requires integrating multiple energy carriers (e.g., power, heat, gas) and engineering domains (e.g., control, ICT) to optimise resource use and enhance resilience. The increasing share of non-programmable renewables, such as solar and wind, introduces grid stability challenges, including voltage fluctuations and congestion, necessitating advanced flexibility solutions. Multi-energy and multi-domain systems address these challenges. This chapter showcases innovative methods and approaches for assessing sector coupling and multi-energy systems in the context of the energy transition.