Nano-photonics in the previous chapters was based on the optical behaviors within the dielectric materials, while metal optics-based surface plasmon (SP)Surface plasmon (SP) in this chapter is involved in another nano-photonic family—a composite electromagnetic (EM) wave that interacts with coherent free electron motions in the vicinity of the metal surface. It allows a small amount of energy loss within the narrow vicinity of the metal surface. Nevertheless, modern interest in the SP is rapidly growing because the EM field in the SP is strongly confined within several tens of nm at the metal/dielectric interface. The result allows the SP to be free of diffraction limit with minimum energy loss, thus leading to a contemporary favorite together with Si photonics and PCs for the next-generation nano-photonic devices. This chapter surveys fundamentals of the SP: first, Drude model-based metal optics leading to a plasmon-polariton (PP) and its dielectric function and dispersion relation, second, basic physics of the SP including a dispersion relation and several physical phenomena originated from the PP, third, an excitation method of the SP, and fourth, a localized surface plasmon (LSP) confined in the metallic nano-particle and nano-gap. Applications of the SPs are introduced in Chap. 7 .