In radio frequency technology (RF technology), the use of scattering parameters for the design of circuit components has been established for decades. Scattering parameters are used in the design as a result of the circuit and field simulation. After setting up an RF circuit, the scattering parameters are measured using a network analyzer and compared with the target values of the simulation. This circuit theoretical description with scattering parameters is also the focus of this chapter. In this chapter on circuit fundamentals, the necessary tools for the development of source-free, linear and time-independent RF circuits are presented. Therefore, the system requirements are briefly described at the beginning. The scattering matrix and the circuit design of second ports via the scattering parameters are then presented for a quick introduction to the practice. The scattering parameter properties of two- and multi-ports are explained in detail and the usefulness of transmission and chain parameters is presented at the end of the chapter.