This chapter covers multi-energy systems (MES) and how they can be used in distribution networks. The focus is on how energy conversion systems in multiple sectors (electricity, heating, cooling, transportation, and gas) can be used to provide flexibility and grid services using demand response (DR). The use of storage (thermal, electrical, compressed air, electrochemical, pumped hydro, etc.) is discussed as a method to provide flexibility in the different energy sectors, taking the regulatory framework and perspectives for MES design, planning, and associated architecture, into account. Finally, techno-economic, market and regulatory barriers, and solutions to the adoption of multi-energy systems are described. The future energy system will require a more integrated and dynamic interaction across all value chains, connecting specific energy resources to end-use sectors. This “System of Systems” approach envisions electricity as the primary energy carrier, with power grids serving as the backbone for decarbonizing all energy sectors. This integration will enable society to realize the full benefits of a multi-energy system. Multi-energy systems (MES) form a fundamental concept in the transition to a low-carbon, integrated, and flexible energy future. By coupling electricity, heat, gas, and fuel systems, MES improves the reliability and efficiency of renewable energy utilization, enables the decarbonization of hard-to-abate sectors, and fosters innovation in energy storage and infrastructure. For engineers, a thorough understanding of MES is essential—not only for effective systems integration, but also for designing robust, scalable, and sustainable energy solutions across all sectors.