Recently sum-frequency has become a very powerful experimental technique in studying structures and dynamic of molecules on surface or molecules between surfaces.In this talk after brief discussion of the derivation of SFG,various possible types of vibrational and electronic SFG’s will be discussed including the derivations of their quantum mechanical expressions based on the Born-Op- Recently sum-frequency has become a very powerful experimental technique in studying structures and dynamic of molecules on surface or molecules between surfaces. This talk after brief discussion of the derivation of SFG, various possible types of vibrational and electronic SFG’s will be discussed including the derivations of their quantum mechanical expressions based on the Born-Op-