Surface plasmon polaritons are propagating energy waves at a plane metallic–dielectric interface. Metallic nanoparticles supply an effective restoring force to drive electrons in an oscillating electromagnetic field. A specific resonance condition leads to a field amplification localized in the near zone inside and outside the particle. This resonance is called a localized surface plasmon resonance (LSPR). In contrast to propagating SPPs, where the momentum-matching coupling techniques are employed, the LSPR can be excited by a direct illumination of electromagnetic waves on the curve surface of nanoparticles. The LSPR give rise to strongly enhanced field amplitude. The radiative decay of the LSPRs results in strongly enhanced scattering in the far field. Nonradiative LSPR absorption also leads to a strong extinction at the resonance frequency. The spectral properties of the LSPRs are sensitive to the geometrical shape, dimensional sizes, and compositional materials of the nanostructure. This chapter begins by exploring the complex physical mechanisms involved in LSPRs. A quasi-static approximation is assumed in analyzing the interaction between electromagnetic waves and nanoparticles with dimensional sizes smaller than the wavelengths. This chapter also discusses the extinction efficiency and spectral properties of nanoparticles. The dielectric surroundings determine the optical properties of a nanoparticle. The subsection on spectral response focuses on the LSPRs with a variety of geometry shapes and dimensional sizes, as well as the interactions among multi-particles. The striking consequences of the resonantly enhanced absorption and scattering of nanoparticles are their fundamental applications in many fields, e.g., emission enhancement and biosensing, which are briefly introduced at the end of the chapter.