 Phosphorus-based flame retardants have been used for decades as effective alternatives to traditional halogens in polymers and other materials. These flame retardants work either by forming intimescent layers when heated or by inhibiting combustion in the gas phase. The effectiveness of these flame retardants depends on their interactions with the polymeric material and any additives present. Additionally, the presence of synergists or adjuvants can further enhance the performance of the flame retardant. Understanding how these factors affect the performance of the flame retardant is essential for developing tailor-made solutions. This article was authored by Bernhard Schartel.