Real life systems are studied by means of input–output relationships known as “transfer functions.” Inherently, these are functions of complex variables commonly presented in the form of Bode magnitude (a.k.a. amplitude, module, gain) and phase (a.k.a. angle, argument) plots. The two-graph combination is commonly used in the fields of control theory and electrical engineering. Hand analysis of these nonlinear functions is based on piecewise linear approximations that proved to be surprisingly simple and accurate. In this chapter, fundamental forms of complex transfer functions are introduced.