Abstract: The evolution of galaxies is closely tied to cyclic feedback processes between stars and the interstellar medium (ISM). Radio recombination lines (RRLs) provide insight into the physical conditions of the ISM, star formation, and the impact of stellar feedback. In starburst galaxies, RRLs illuminate young, massive clusters, probing the early evolution of this highly efficient mode of star formation. When observed at low (<5 GHz) radio frequencies, RRLs arise from the diffuse interstellar medium (ISM), tracing diffuse ionized gas and the elusive cold neutral medium. Using low frequency radio spectroscopy, we have been showing that RRLs (i) can be detected outside of the local universe (e.g., z~1), (ii) constrain key physical properties of the ISM clouds, and (iii) bring the community a new way to investigate the evolution of the ISM in galaxies and active galactic nuclei (AGN). In this talk I will highlight the key science made possible using observations of RRLs in external galaxies and discuss the exciting future ahead.