The general integral properties of the dispersion theory and sum rules in linear and nonlinear optics were discussed. The response of the system to the electromagnetic perturbation through the formalism developed by Kubo was also studied. Investigations show the linear and nonlinear susceptibilty from the simple classical oscillator models. The sum rules were used to check the approximation theory of nonlinear phenomena. The results show the role of dispersion theory and sum rules in the expanding field of nonlinear optics.