Plasmonics involves the localization and guiding of electromagnetic energy in metal/dielectric structures, with applications in sensing and waveguiding for photonic devices. The review discusses localized plasmon resonances in metallic nanoparticles, which enable enhanced light fields for Raman scattering and nonlinear processes. Interface plasmon polaritons (SPPs) propagate along flat boundaries of thin metallic films, with applications in waveguiding along patterned films, stripes, and nanowires. Interactions between plasmonic structures and optically active media are also explored. Localized plasmon resonances in metallic nanoparticles arise from coherent electron oscillations, leading to strong optical absorption and scattering. These resonances are influenced by particle size, shape, and surrounding medium. For nanoparticles much smaller than the wavelength of light, the quasistatic approximation is valid, with the polarizability determined by the dielectric function of the metal. Larger particles exhibit broader resonances due to retardation effects and higher-order modes. Interacting particle ensembles, such as ordered arrays of noble-metal nanoparticles, enable collective plasmon resonances. These arrays can guide electromagnetic energy over several hundred nanometers via near-field interactions, with applications in nanoscale optical networks. Randomly nanostructured metallic surfaces can also produce "hot spots" of extreme field enhancement, useful for single-molecule spectroscopy. Interface plasmon polaritons propagate along metal/dielectric boundaries, with SPPs confined to one dimension. These can be excited using various techniques, including prism coupling and surface gratings. SPPs have applications in evanescent surface sensors and higher harmonic generation. For thin metal films, surface-plasmon polaritons can propagate over long distances, with attenuation lengths ranging from tens of microns to hundreds of microns. Metallic stripes and nanowires offer two-dimensional confinement of electromagnetic energy, with nanowires significantly smaller than the diffraction limit. These structures can guide electromagnetic energy with mode confinement below the diffraction limit, enabling efficient end-fire coupling to optical fibers. Apertures in metallic screens allow light transmission through subwavelength holes, with plasmon-assisted evanescent tunneling playing a key role. This phenomenon has been observed in both regular and single apertures, with corrugated surfaces enabling beam steering and wavelength-selective lenses. Interactions between plasmonic structures and optically active media, such as quantum dots and rare-earth ions, enhance optical processes. SPPs can significantly increase the overlap between optical modes and gain media, enabling reduced cladding heights in quantum cascade lasers. However, the non-radiative decay rate of emissive species near metallic surfaces poses challenges for plasmonic cavity design. The field of plasmonics is rapidly growing, with ongoing research into numerical design tools, fabrication techniques, and near-field optical characterization. The integration of plasmonic components into optical chips and devices is a promising area ofPlasmonics involves the localization and guiding of electromagnetic energy in metal/dielectric structures, with applications in sensing and waveguiding for photonic devices. The review discusses localized plasmon resonances in metallic nanoparticles, which enable enhanced light fields for Raman scattering and nonlinear processes. Interface plasmon polaritons (SPPs) propagate along flat boundaries of thin metallic films, with applications in waveguiding along patterned films, stripes, and nanowires. Interactions between plasmonic structures and optically active media are also explored. Localized plasmon resonances in metallic nanoparticles arise from coherent electron oscillations, leading to strong optical absorption and scattering. These resonances are influenced by particle size, shape, and surrounding medium. For nanoparticles much smaller than the wavelength of light, the quasistatic approximation is valid, with the polarizability determined by the dielectric function of the metal. Larger particles exhibit broader resonances due to retardation effects and higher-order modes. Interacting particle ensembles, such as ordered arrays of noble-metal nanoparticles, enable collective plasmon resonances. These arrays can guide electromagnetic energy over several hundred nanometers via near-field interactions, with applications in nanoscale optical networks. Randomly nanostructured metallic surfaces can also produce "hot spots" of extreme field enhancement, useful for single-molecule spectroscopy. Interface plasmon polaritons propagate along metal/dielectric boundaries, with SPPs confined to one dimension. These can be excited using various techniques, including prism coupling and surface gratings. SPPs have applications in evanescent surface sensors and higher harmonic generation. For thin metal films, surface-plasmon polaritons can propagate over long distances, with attenuation lengths ranging from tens of microns to hundreds of microns. Metallic stripes and nanowires offer two-dimensional confinement of electromagnetic energy, with nanowires significantly smaller than the diffraction limit. These structures can guide electromagnetic energy with mode confinement below the diffraction limit, enabling efficient end-fire coupling to optical fibers. Apertures in metallic screens allow light transmission through subwavelength holes, with plasmon-assisted evanescent tunneling playing a key role. This phenomenon has been observed in both regular and single apertures, with corrugated surfaces enabling beam steering and wavelength-selective lenses. Interactions between plasmonic structures and optically active media, such as quantum dots and rare-earth ions, enhance optical processes. SPPs can significantly increase the overlap between optical modes and gain media, enabling reduced cladding heights in quantum cascade lasers. However, the non-radiative decay rate of emissive species near metallic surfaces poses challenges for plasmonic cavity design. The field of plasmonics is rapidly growing, with ongoing research into numerical design tools, fabrication techniques, and near-field optical characterization. The integration of plasmonic components into optical chips and devices is a promising area of