This review discusses how low-energy, valence excitations created by swift electrons can provide unmatched spatial resolution for studying the optical response of structured materials. Electron microscopes can focus electron beams on sub-nanometer spots and probe the target response through energy loss or emitted radiation. Theoretical frameworks for calculating energy loss and light emission (cathodoluminescence) are revisited and compared with experimental results. A quantum-mechanical description of electron-sample interaction is discussed, followed by a classical dielectric approach applicable to complex systems. The conditions under which classical and quantum formulations are equivalent are assessed. Collective modes like plasmons in bulk materials, planar surfaces, and nanoparticles are studied. Light emission from electrons is shown to be an excellent probe of plasmons, combining sub-nanometer resolution with nanometer resolution in wavelength. Both electron energy-loss and cathodoluminescence spectroscopies in scanning mode yield detailed images of plasmon modes in nanostructures, making them ideal for nanophotonics studies. The review covers the interaction of swift electrons with matter, including evanescent light sources, classical dielectric formalism, and quantum approaches. It discusses electron energy-loss spectroscopy, focusing on space, momentum, and energy resolution, bulk losses, planar surfaces, and complex geometries. It also explores cathodoluminescence, mechanisms of light emission, and related phenomena. The review highlights the potential of electron microscopy for plasmonics and nanophotonics, emphasizing its ability to achieve high spatial and energy resolution. The review concludes with a discussion of the prospects for plasmonics and the importance of electron microscopy in advancing nanophotonics studies.This review discusses how low-energy, valence excitations created by swift electrons can provide unmatched spatial resolution for studying the optical response of structured materials. Electron microscopes can focus electron beams on sub-nanometer spots and probe the target response through energy loss or emitted radiation. Theoretical frameworks for calculating energy loss and light emission (cathodoluminescence) are revisited and compared with experimental results. A quantum-mechanical description of electron-sample interaction is discussed, followed by a classical dielectric approach applicable to complex systems. The conditions under which classical and quantum formulations are equivalent are assessed. Collective modes like plasmons in bulk materials, planar surfaces, and nanoparticles are studied. Light emission from electrons is shown to be an excellent probe of plasmons, combining sub-nanometer resolution with nanometer resolution in wavelength. Both electron energy-loss and cathodoluminescence spectroscopies in scanning mode yield detailed images of plasmon modes in nanostructures, making them ideal for nanophotonics studies. The review covers the interaction of swift electrons with matter, including evanescent light sources, classical dielectric formalism, and quantum approaches. It discusses electron energy-loss spectroscopy, focusing on space, momentum, and energy resolution, bulk losses, planar surfaces, and complex geometries. It also explores cathodoluminescence, mechanisms of light emission, and related phenomena. The review highlights the potential of electron microscopy for plasmonics and nanophotonics, emphasizing its ability to achieve high spatial and energy resolution. The review concludes with a discussion of the prospects for plasmonics and the importance of electron microscopy in advancing nanophotonics studies.