This book is devoted to Gas-Phase Thermal Reactions (GPTRs), and especially combustion reactions, which take place in engines, burners and industrial chemical reactors to produce mechanical or thermal energy to incinerate pollutants or to manufacture chemical substances, and which play an important part due to the consequences they have on the environment : fires and explosions, tropospheric pollution, greenhouse effect, hole in the stratospheric ozone layer. The design and running of engines, burners, incinerators, industrial reactors, both economical in fuels, raw materials and energy, efficient, safe and clean, as weIl as the scientific evaluation of the causes and the effects of atmospheric pollutions with a view to taking rational environmental decisions, which necessitate an understanding of the fundamental mechanisms of these reactions and an access to models allowing numerical simulations of the phenomena being studied to be carried out. The analysis of the results ofthe simulations then allows an optimal solution to be found to the industrial problem or to extrapolate the natural phenomena.