In star-forming galaxies, from systems like our own Milky Way to high-redshift disks, most of the gaseous reservoir is in the atomic and molecular ISM. This gas is supported against gravitational collapse primarily by turbulence. Losses from turbulent dissipation and radiation must be constantly replenished to prevent runaway star formation. Yet, star formation itself is crucial to maintaining the ISM’s equilibrium, as feedback from short-lived massive stars — including UV radiation and supernovae — is the primary energy source. I will discuss theory and numerical hydrodynamic/RHD simulations that quantify the physics of feedback. Resolved simulations provide a close-up view and calibration of processes that are sometimes treated via subgrid models in galaxy formation simulations. We find that radiation forces can be important to ejecting gas and limiting the efficiency of individual star-forming clouds. However, supernovae play the most important role in the ISM overall, because the momentum injected by Sedov-Taylor blast waves is an order of magnitude greater than other source terms. Resolved simulations show that each supernova blast robustly provides momentum ~ 1-4 e5 Msun km/s to the ISM. This level of momentum input is just what is required to explain ISM properties and to self-regulate star formation rates as observed in a wide range of galactic environments.