Publisher Summary Surface plasmon polaritons are electromagnetic modes with a locally enhanced electric field. These modes are expected to become the key for the development of photonics of the 21st century and thus the applications of surface plasmon polaritons have become a worldwide target to be studied. In particular, localized surface plasmons (LSP's) have been widely studied as a key electromagnetic mode to develop nano-photonic technology. One of the benefits of mastering the physical properties of the LSP comes from its application to nonlinear optics. Little is known of nonlinear optical response of the LSP, although it is quite significant to develop optical devices, for example, an optical switch, based on nonlinear optical phenomena because of the LSP excitation. This chapter has succeeded to present the phenomena of optical switching and optical bistability by using a modified nonlinear Mie theory for a Ag nano-sphere coated with a CdS film. This simulation takes a long time and is achieved along a radius axis. It is therefore not easy to understand circumstances over the whole of the concentric shell. To overcome this disadvantage, the nonlinear optical response of a Ag sphere coated with CdS films is demonstrated using finite-difference time domain (FDTD) simulations. This chapter first introduces numerical results of the nonlinear optical response of a Ag sphere coated with CdS films. Secondly, the fabrication process of a Ag sphere coated with CdS films is described. Thirdly, the experimental results of the linear and the nonlinear optical responses of samples fabricated are given.