The progress in the PMO theory made by Dewar, Hoffmann, Epiotis, Houk, Wolfe and Kost et al, is reviewed. The various PMO methods have been used as the useful instrument for analyzing the effects of the orbital interactions on the molecular geometry and on the sterospecific and regiospecific reactions, such as 1, 2-asymmetric addition to carbon-oxygen double bond of ketone and various cycloaddition reactions. Epiotis' Non-Bonded Orbital interaction theory has made organic chemist to understand that a crowded geometry is not more aberration of an order dominated by "steric effect". The PMO analysis for the π orbital interaction shows that the interaction between the conjugated groups is repulsive in nature. Our WSW-like procedure provides for the quantitative PMO analysis a set of complete fragment MOs basis, and it has succeed in transforming the orbital interaction energy into the atomic interaction energy.

Key words: ORGANIC CHEMISTRY, PERTURBATION MOLECULAR ORBITAL