For the past decade, various nonlinear optical processes for molecular systems have been widely investigated both experimentally and theoretically. These processes can be described by hyperpolarizability. The hyperpolarizability is also one of the most essential factors which characterize various molecule-molecule interactions. However, high-order nonlinear optical processes are generally complicated and then any intuitive pictures of hyperpolarizability have not been presented. Our objective is to elucidate the mechanism of the second-and the third-order nonlinear optical processes and to obtain physical and chemical pictures of the ability of high-order electron fluctuation, which is an essential factor of high-order polarization. To this end, we have developed some calculation and analysis methods and have proposed a new concept “hyperpolarizability density”. The analysis by using this concept enables us to extract a classical intuitive pictures of hyperpolarizability. The features of hyperpolarizabilities for various molecular systems are investigated in relation to the virtual excitation processes and the resonance structures. On the basis of these analyses, a structure-property correlation rule is extracted.