District energy systems are transitioning from conventional steam and chilled-water networks to highly integrated thermal energy networks and multi-energy systems that couple heating, cooling, electricity, storages, renewable resources, and disparate sectors. This lecture presents how Modelica enables the development of scalable, physics-based models for the design, operation, and optimization of these increasingly large and networked systems. Through case studies in steam district heating, district cooling, and integrated community energy systems, students will learn how Modelica can be used to address challenges in thermo-fluid modeling, optimal control, and system design across multiple spatiotemporal scales. The presentation concludes with emerging opportunities in resilient, circular, and biomimetic infrastructures.