we discussed the techniques for the modification of polymer properties based on structural modification of polymer either during or post polymerization. Even though structural modification of polymers often leads to significant property modification, very few polymers are used technologically in their chemically pure form; it is generally necessary to modify their behavior by the incorporation of additives. Additives are usually required to impart stability against the degradative effects of various kinds of aging processes and enhance product quality and performance. Thus many commercial polymers must incorporate thermal and light stabilizers, antioxidants, and flame retardants. In addition to these additives that influence essentially the chemical interaction of polymers with the environment, other additives are usually employed to reduce costs, improve aesthetic qualities, or modify the processing, mechanical, and physical behavior of a polymer. Such additives include plasticizers, lubricants, impact modifiers, antistatic agents, pigments, and dyes. These additives are normally used in relatively small quantities; however, nonreinforcing fillers are employed in large quantities to reduce overall formulation costs provided this does not result in significant or undesirable reduction in product quality or performance. In some cases, a given polymer may still not meet the requirements of a specific application even with the incorporation of additives. In such cases, the desired objective may be achieved through alloy formation or blending of two or more polymers. In this chapter, we discuss the upgrading of the performance of polymers through the use of additives and reinforcements.