Polymer composites are composed of a polymer matrix in which another well-defined material with distinctive phase boundary is dispersed. Hence, the thermal properties of polymer composites depend on both the macromolecular matrix and the additives. Thermal analysis methods have proved useful not only in defining suitable processing conditions for these materials and drawing up useful service guidelines for their application, but also for obtaining information on thermal properties polymer structure relationships. There is currently a high level of interest in using nanoscale reinforcing fillers for the preparation of polymeric nanocomposite materials with exceptional properties. An improvement in the thermal stability and flammability properties of polymers has been obtained with nanoscale additives, and these filled systems provide an alternative to conventional flame retardants. It is therefore important to explore how the asymmetry (aspect ratio) and other geometrical effects of nanoadditives influence the thermal degradation properties of polymer nanocomposites. Despite its significance, the topic of nanocomposites has only been highlighted in this work through a few representative cases and readers are directed to current specialised reviews available in the literature [a.252, a.253]. Nevertheless, the thermal degradation of some of the most common reinforced polymer composites is presented in the following sections.