This presentation 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. Both spontaneous (auto) and piloted (forced) ignition phenomena will be 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 theoretical model is formulated and specific numerical and analytical solutions are discussed in the light of experimental evidence and data. It appears that within the approximation of constant surface temperature at ignition, the ignition delay data may be correlated by a simple thermal model based on inert heating of the solid. However, a significantly more complicated description which includes gas and solid-phase chemistry is required if the surface temperature at ignition is not constant.