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.