 Phosphorous-containing additive and reactive flame retardants are increasingly successful as halogen-free alternatives for various polymeric materials and applications, acting in both the condensed and gas phases to enhance charring, inhibit flames, and form inorganic glass. Flame retardancy is dependent on the flame retardants' interaction with pyrolyzing polymeric material and additives, and can be modified through the use of synergists slash adjuvants and changes to the polymeric material. A detailed understanding facilitates tailored and targeted development. This article was authored by Bernhard Schartel.