Here the interaction between Rydberg atoms and cavity photons is explored. Starting with the JaynesCummings model, the chapter extends to phenomena specific to Rydberg excitations inside cavities, such as self-induced Rabi oscillations, vacuum-field susceptibility, and blackbody-induced decoherence. Photon Fock states, the operation of a Rydberg maser, and cavity-mediated ultrafast energy exchange are analyzed. Special focus is given to experimental schemes coupling cold Rydberg atoms with cavities, including two-photon excitation and challenges in free-space excitation, emphasizing the potential for strong light-matter coupling at the few-photon level.