This chapter summarizes basic properties of the main Surface plasmon polariton (SPP) modes. SPPs are electromagnetic (EM) excitations that, being coupled to surface collective oscillations of free electrons in a metal, are bound to and propagate along metal-dielectric interfaces. Light-matter interactions in nanostructured materials give rise to nanostructured optical fields, implying that EM fields of SPP modes supported by metal nanostructures exhibit subwavelength spatial frequencies. The SPP electric field is thereby primarily transverse in the dielectric and longitudinal in the metal, reflecting the hybrid nature of SPPs that combine the features of both propagating EM waves in dielectrics and free electron oscillations in metals. The SPP waveguide configuration representing a finite-width metal stripe placed on a dielectric substrate is probably the most natural and certainly the best studied one to date. SPPs represent interface EM excitations, in which evanescent EM waves are coupled to surface collective oscillations of free electrons, propagating along metal-dielectric interfaces.