The rate processes involved in the soot and PAH formation from combustion are introduced and discussed. Firstly, the nucleation process of the carbon particle is discussed in comparison with the classical theory of homogeneous nucleation used for the atmospheric cloud condensation and the industrial fine particle production. The simple concept of the critical radius does not seem to be directly applicable for the soot nucleation. The potential improvement of the particle formation model is discussed based on the observed physical and chemical properties of the diesel soot. Secondly, the major reaction mechanisms of PAH formation and growth including the popular HACA mechanism are introduced with a special attention to the recombination reactions of resonantly stabilized hydrocarbon radicals, which is a class of reactions with potentially new and significant routes. The chemical and thermodynamic roles of the resonance stabilization of hydrocarbon radicals are introduced and discussed.