This chapter presents the thermodynamics of combustion. The mass conservation of the elements is applied to balance the global combustion reaction of stoichiometric and lean mixtures. In this context, the concepts of stoichiometric, lean, and rich mixtures are introduced, together with the definition of the equivalence ratio. Also, energy conservation is applied to determine the adiabatic flame temperature of global combustion reactions, considering constant pressure and constant volume processes. The approach is demonstrated for constant heat capacities and for variable enthalpies. The equilibrium condition is applied to study incomplete combustion reactions, which are characteristic of rich mixtures. The equilibrium condition is applied to the products by following two methods, namely, the method of the equilibrium constant and the Gibbs free energy minimization method. The latter method is more adequate to analyze global reactions with many species in the products. The examples presented in this chapter show the application of thermodynamics theory to the combustion processes of hydrogen, ammonia, and ethanol.