Light emission by ensembles of emitters can be tailored using resonators such as cavities or plasmonic antennas. While concepts such as field enhancement, Purcell effect, and quenching can be used to understand the interplay between a two‐level system and a resonator, they fail to account for light emission by ensembles of emitters. Recent experiments reporting light emission by thermalized molecules, quantum dots, and hot electrons excited optically or electrically are reviewed. It is shown that the local Kirchhoff's law provides a unified framework to discuss photoluminescence, electroluminescence, and thermal radiation by ensembles of thermalized emitters coupled to resonators.