In the previous chapter, we saw that a metal nanoparticle (NP) can generate localized surface plasmon resonance (LSPR), as a result of the resonance of free electrons on a curved surface. We also mentioned the main difference between surface plasmon polaritons (SPP) and LSPR, namely that SPPs are propagating waves, while LSPR is a nonpropagating excitation of free electrons in a metallic nanostructure coupled to an EM field. In this chapter, we will see that SPPs in restricted geometries (such as nanoantennas or nanocavities) can also generate LSPR. Nevertheless, the conditions for excitation are different. LSPR can be excited by direct application of light, whereas SPPs can be excited by matching the frequency and momentum of the excitation light and the SPPs. For example, under laser illumination, an antenna develops a strong dipole along the antenna. Charge oscillations inside each arm give rise to strong EM fields inside the feedgap of the antenna. When the antenna is resonant at optical frequencies, it can be called a resonant nanoantenna. In this case, with sufficiently close spacing with respect to a quantum emitter (QE) and nanoantenna, they can interact forming an electric dipole.On the other hand, the extremely high localized fields in plasmonic nanocavities or plasmonic resonators, although they generally have low quality factors Q, are able to confine modes to sub-wavelength volumes V. Thus, plasmonic nanocavities become much more competitive with conventional optical resonators for applications where a resonance confined to a small volume is desirable. Hence, a nanocavity can profoundly change the number of EM modes and decay pathways available to a QE, increasing or decreasing its radiative decay rate. In both cases (nanoantenna and nanocavity), we can manipulate the local density of optical states (LDOS) of the QE, and increase the efficiency of light emission from the QEs.