This chapter examines the extensive use of modular approach for simulation, modeling, optimization, and control of energy systems. The chapter first examines the bottom-up scientific approach for modular simulation and modeling. This includes standardized modular design—Aspen platform and others. The chapter subsequently examines the top-down modular approach for global techno-econometric/social model of energy systems. These bottom-up and top-down approaches have led to the development of full-scale hybrid models using soft and hard links to connect bottom-up and top-down models. These linkages have resulted in the development of numerous types of hybrid models depending on the end goals. The hybrid models can also be large-scale fully integrated models. Finally, the chapter describes the large-scale modeling system NEMS (National Energy Management System) which combines multiple modules. The chapter ends with brief reviews of literature for five topics: modular simulations of energy system equipment, modular simulation of building construction and energy consumption, examples of modular approach for renewable power supply, modular simulation of energy consumption and efficiency, and novel modular approaches for simulation and optimization.