Nanoparticles can reduce the flammability of polymers because of the reduction in heat release rate, the increase in flame-out and their ability to auto-extinguish. The mechanisms involve physical barrier effects and catalytic processes, which can modify the degradation of polymers and form charred protective layers reinforced by the nanoparticles. The properties conferred by nanoparticles depend on dispersion and the chemical modifications to improve compatibility with polymers. The reduction of total heat released during combustion is achieved by a combination of nanoparticles and flame retardants. Future trends include the development of nanoparticle/polymer hybrids and the optimization of the composition and morphology of the protective barrier.