Unstabilized polyolefins are prone to several degradation reactions during the different stages of compounding/processing/fabrication, with catastrophic consequences on product stability and in-service performance. The level of protection afforded by a stabilizer system against the deleterious effects of mechanical, thermal, thermo-, and photoxidative degradation reactions is governed by several factors which include both physical and chemical characteristics of the substrate and stabilizer, the environment, and the application of stress. In considering the practical aspects for the selection of a stabilizer system, due regards must be given to the structural nonhomogeniety of polyolefins, and to stabilizer substantivity and stability as well as its intrinsic activity. The stabilizer function is examined here in the light of current understanding of the behavior of functions of antioxidants and stabilizers. Several examples are included to reinforce the mechanistic aspects of different classes of antioxidants. Further, growing demands for higher performance polyolefins for medical, food, telecommunication, automotive, and building applications places increasing demands on stabilizers and antioxidants under hostile and aggressive environments. In-situ binding of stabilizers is a practical approach which improves the performance of both the polymer and the stabilizer.