The interplay of free electrons, light, and matter gives rise to many intriguing electromagnetic phenomena that can occur on length scales far below the optical diffraction limit and on time scales shorter than an optical cycle. In this thesis, we explore the vast potential of optical metasurfaces, nano-textured materials with artificial optical properties, to manipulate and control these phenomena at will. As a starting point, we apply electron energy-loss spectroscopy (EELS), cathodoluminescence (CL) spectroscopy, and photon-induced near-field electron microscopy (PINEM) to study the fundamental correlation between the spontaneous and stimulated interactions of free electrons and photons in the optical near-field of a chemically-synthesized gold nanostar. Then, we exploit the Smith-Purcell effect to demonstrate metasurfaces that simultaneously generate and shape free-electron radiation in the visible spectral range. Further on, we implement an optical-fibre-integrated metagrating that permits the coherent coupling of free electrons and guided optical modes, rendering a versatile platform to both collect free-electron radiation and structure optical fields that manipulate the electron wave function. Finally, we explore how the manipulation of non-relativistic electrons with light can be probed experimentally, a challenge that has been hardly addressed so far, using an electrostatic retarding field analyser that is retro-fitted into a scanning electron microscope. The insights from this thesis may inspire the design of novel light sources, may enable spectroscopic tools that reveal the dynamics of optical phenomena on ultra-small length and time scales, and, ultimately, may contribute to a deeper fundamental understanding of the electron-light-matter interaction.