In this chapter, we discuss how the circuit component models introduced in Chap. 2 are assembled into a circuit simulation model of an electronic system. A buck converter is used throughout the chapter to illustrate the necessary steps. At the beginning of the chapter, we demonstrate how the functional circuit of the converter is translated into a model suitable for calculating conducted emissions. We examine the impact of parasitics in the buck converter’s components and the interconnects between them. To estimate interconnect properties, we introduce simple analytical equations, without performing EM simulations at this stage. The converter is analyzed using both transient and frequency-domain circuit simulations, and we compare the differences between these approaches. Toward the end of the chapter, we discuss the limitations of this modeling approach and highlight common pitfalls encountered when creating circuit-level models. In the advanced section, we provide additional details on the simulation algorithms used in SPICE simulators.