An electromagnetic wave traveling in time and space can reach a particular target, and then interact with it, as shown in Figure 3.1. As a consequence of this interaction, part of the energy carried by the incident wave is absorbed by the target itself, whereas the rest is reradiated as a new electromagnetic wave. Due to the interaction with the target, the properties of the reradiated wave can be different from those of the incident one. The question that arises at this point is if these changes could be employed to characterize or identify the target. In particular, we are interested in the changes concerning the polarization of the wave. In the following, we describe the interaction between an electromagnetic wave and a given target.