Contrary to the mechanical waves, the electromagnetic ones are transverse waves, introducing a new dimension in optical phenomena: polarization. Numerous living organisms, endowed with photosensitive sensors, were found to use this component of light. It seems that most insects are sensitive to linear polarization of light, that is, they are capable of distinguishing two different states of polarization, which is not the case with mammals, primates, and hominids. Concerning humans, this natural handicap turns out to be a source of bio-inspired innovations, because we know how to reveal, by means of simple instruments, what is a priori hidden. Polarization is one of the ways to fight against the mimicry, and even in nature, many animals, butterflies in particular, actually use it (Fig. 17.1). In this chapter, we will present the main types of photonic structures of insects that present such polarization effects. Fundamental recalls on the polarization of the light are presented in much more detail in the Annex of this chapter.