Our understanding of galaxy evolution centers around questions of how gas gets into galaxies, how it participates in star formation and black hole growth, and how it is returned to its galactic surroundings via feedback. I will present observational results on the relationship between gas that forms stars and gas that accretes onto supermassive black holes, and the nature of feedback that is capable of removing gas from galaxies. These results have important implications for how radiation, momentum, and kinetic energy from stars and black holes regulate the cold gas supply in galaxies. I will also discuss prospects for characterizing the physical properties of gas in and around galaxies using multi-wavelength spectroscopy with existing and future facilities.