A wide variety of additives are used in plastics to enhance their performance and increase their versatility. This chapter describes all the major additives used, apart from fillers which are considered separately. The function and application of each type of additive are described, and typical products are discussed. Antioxidants protect polymers from the harmful effects of oxidation. Coloration is explained and current practices are described. Plasticizers and lubricants are discussed briefly, but considered in more detail elsewhere in view of their very particular application to polyvinyl chloride. Types of impact modifiers used to toughen glassy polymers are described. Most plastics readily ignite and burn with a high heat output, so the fire or flame retardant additives which are very important ingredients in many of their applications are considered. Blowing agents essential for foam production are also discussed. A final section considers briefly other additives used for more specific purposes, including cross-linking additives, surface modifiers, compatibilizers, viscosity modifier, biocides, and nucleating agents.