Halogenated, brominated, and chlorinated organic flame retardants are widely used to reduce the flammability or ignitability of plastics; this use is growing because of increasingly strict flammability regulations. Brominated and chlorinated aliphatic and brominated aromatic compounds are used as additives to both thermoset and thermoplastic polymers. Brominated aliphatic, brominated aromatic, and chlorinated aromatic compounds are used as reactive flame retardants in both thermoset and thermoplastic polymers. In many applications, antimony oxide is used as a synergist, interacting with halogenated flame retardants to produce a more effective system. The largest volume brominated flame retardants are the brominated aromatics tetrabromobisphenol A and decarbromodiphenyl oxide. The largest volume chlorinated flame retardants are the chlorinated paraffins. The toxicity of these materials is discussed.