This chapter discusses circuit component models used throughout this book and their properties. We interpret circuit models as impedances, which are used in SPICE simulations to solve circuit simulation tasks. The introduction of circuit components follows the decomposition of an EMC problem into three key elements: source, coupling path, and sink. First, we present circuit-level source models, followed by a discussion on modeling the coupling path in circuit simulations. We introduce the concept of electrical size and explore network parameter models, which provide an efficient approach for representing electrically large coupling paths within a circuit simulation. Next, we examine sink models, including generic sinks and LISNs commonly used in EMC testing. Although not strictly a circuit-level sink but rather a post-processing step, we also introduce the principles of EMI receivers in simulations. In the advanced sections, we discuss the linearity of circuit components and explain how lumped models can be derived from network parameter data.