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This paper discusses the mechanisms that lead to ignition of fires and the reasons behind the experimental correlations available in the literature. The objective is to understand and quantify the physics of heat and mass transfer and the chemistry of solid–phase decomposition and gas–phase runaway reactions that result in the appearance of a sustained gas–phase diffusion flame— a phenomenon identified as ignition. Both spontaneous (auto) and piloted (forced) ignition phenomena are discussed. Two types of materials commonly found in building fires are considered— thermoplastics that melt and vaporize upon heating, and cellulosic materials that decompose and produce char. A general theoretical model is derived and specific numerical and analytical solutions are discussed in the light of experimental evidence and data. It is concluded that within the approximation of constant surface temperature at …