Galaxies are sometimes described as "factories" where baryons accreted from the cosmic web are locked up as stars. This transformation is, however, is both much less complete and much slower than would be naively expected. Much of the baryonic contents of the Universe is still in gaseous form, both in galaxies and in the circumgalactic medium. The reason star formation is so slow is that the energy returned by massive, luminous stars to their environment limits the rate of gravitational collapse, and even ejects material from galaxies as “super winds.” In this talk, I will discuss how theory and radiation magnetohydrodynamic simulations that simulate the interstellar medium (ISM) at high resolution have been able to explain how galaxies work. I will present results characterizing the effects of stellar feedback including stellar UV radiation, winds, and supernovae, and showing how a match between the pressure “yield” from feedback and the weight of the ISM leads to an equilibrium. In this equilibrium, star formation and the properties of the interstellar medium are co-regulated, allowing for the widely ranging states seen in both nearby and distant galaxies.