Based on their response to temperature, plastic materials may be classiﬁed into two main categories: thermoplastics and thermosets. A thermoplastic behaves like a ﬂuid above a certain temperature level, but the heating of a thermoset leads to its degradation without its going through a ﬂuid state. In fact, this reﬂects the behavior of plastic materials under the action of ﬁre, providing the simplest identiﬁcation test. This classiﬁcation is not restricted to plastic materials but may also be extended to the behavior of coatings, adhesives, and several other categories. This is why we ﬁnd it better to use the term thermosetting polymers, which implies the different ways in which these materials are used and adds the fact that a constitutional repeating unit (CRU) is present in their chemical structures. These materials are also referred to as thermosetting resins, which is a vaguer deﬁnition that may be applied to the starting monomers or oligomeric precursors, as well as to the ﬁnal materials.