As we said at the beginning of the previous chapter, we take it for granted that the reader has been previously exposed to the fundamentals of circuit and network theory. In particular, we assume that the reader is familiar with the representation of time-harmonic (i.e., varying sinusoidally in time) scalar quantities by means of complex numbers, i.e., with the so-called Steinmetz method (or procedure). This assumption is based on typical electrical engineering and/or electronics curricula, and also applies to curricula in physics in many universities. Those readers for whom this assumption does not apply can grasp the fundamentals of Steinmetz procedure in the next section. However, it is highly recommended that they read books that treat this subject in depth, and especially that they familiarize themselves with the Steinmetz method by solving many problems on circuits. For those readers who, on the contrary, feel confident that they know the procedure, the first part of this chapter may look rather tedious. Nevertheless, we recommend that they read carefully at least the second part, beginning with Section 2.5, as otherwise some subtle details in the following chapters might be difficult to grasp.