The process whereby large quantities of energy and momentum are released into the gas surrounding star formation regions in galaxies. More specifically, massive stars inject energy, mass, and metals back to the interstellar medium through stellar winds and supernova explosions. Feedback inhibits further star formation either by removing gas from the galaxy, or by heating it to temperatures that are too high to form new stars. Observations reveal feedback in the form of galactic-scale outflows of gas in galaxies with high star formation rates, especially in the early Universe. Feedback in faint, low-mass galaxies probably facilitated the escape of ionizing radiation from galaxies when the Universe was about 500 million years old, so that the hydrogen between galaxies changed from neutral to ionized, a process called reionization.