The dynamics of photo-induced non-adiabatic reaction can be treated by quantum mechanics in much the same way as one would treat an adiabatic reaction. Meanwhile, various semi-classical approaches have been developed for treatment of processes of non-adiabatic reactions. The main disadvantage of both quantum and semi-classical methods is that they require a complete specification of a very large amount of information about the potential energy surfaces. For most polyatomic molecules, such accurate surfaces are impossible to obtain from ab initio calculations. This review will focus on quantitative description of photochemical and photophysical processes of a polyatomic molecule, which is based on structures and properties of several critical points of the potential energy surfaces. The improved ab initio molecular dynamics method has been used to simulate the initial relaxation processes from Franck-Condon geometry in an excited electronic state. Statistical non-adiabatic rate theory has been developed for calculating rate constants of inter-system crossing and internal conversion via vibronic interaction. The mechanistic photodissociation of aliphatic, aromatic, and α,β-unsaturated carbonyl compounds have been determined with these ab-initio-based techniques and methods. All of these are included in this contribution.