Mild combustion is an emerging technology in several fields of practical applications, from material treatment to energy conversion as well as to pollutant abatement [1]. In a survey of potential keys for the mitigation of environmental problem, it represents a flexible and clean process that is the result of a trade-off in the optimization of fuel conversion with respect to efficiency (in term of energy saving and pollutant abatement) while not requiring drastic changes in the configuration of traditional plants. In this chapter the attention will be focused on non-premixed combustion processes, thus on configurations where the fuel and the oxidant flows are fed separately, then mix and react. A combination of both heating and dilution of oxidant and/or fuel yields a not premixed combustion process which is named Hot Diluted Diffusion Ignition (HDDI) [2, 4] when • heating contributes significantly to the creation of an oxidative structure in the sense that no combustion process occurs without it; • dilution is so intense that the maximum temperature attainable inside this structure is so low that, in turn, it affects significantly its placement in the mixture fraction domain, the structure itself and the physical and chemical kinetics when compared to a diffusion flame process.