This chapter introduces the motivation for studying Rydberg atoms in the context of cavity quantum electrodynamics (QED). It outlines the unique properties of highly excited Rydberg states, the role of strong and tunable interactions, and the importance of coupling Rydberg ensembles to confined electromagnetic modes. The discussion sets the stage for exploring fundamental phenomena such as blockade physics, quantum nonlinear optics, and cavity-mediated phase transitions, as well as their applications to emerging quantum technologies.