This chapter reviews the main types, mechanisms, and effects of degradation phenomena that may occur during the life of material systems based upon thermoplastics or thermosetting polymers, which can affect their performances in service. Since degradation can be identified with any change in physical or chemical characteristics relative to initial or desirable properties, this generic term embraces any number of reactions that may occur in a polymer. The degradation of polymers may involve several physical and/or chemical processes (e.g., photodegrada-tion, biodegradation, thermal and thermo-oxidative degradation, ultrasonic degradation, high-energy radiation-promoted degra-dation, chemical degradation, mechanical degradation/cracking, electrical aging, etching and corrosive degradation, abrasive

degradation), which promote changes that may lead to a significant deterioration of polymer quality and, hence, to the loss of its functional features. Particular attention here is focused on the role and effect of the presence of micro-and nanofillers on the degradation reactions and on the way by which such nanoscaled additives are able to modify and/or limit degradation phenomena.