Abstract: I will review recent observations and theoretical estimates of the spatial extent of galaxies, defined as systems of stars and gas embedded in extended halos of dark matter and hot gas. Formed by the infall of smaller systems, their sizes are determined by gravitational assembly, gas dynamics, and chemical enrichment in heavy elements blown into extragalactic space by galactic winds. But the full extent of galaxies remains poorly determined. The “virial radius” and “splash-back radius” approximate the separation between collapsed structures and infalling matter. Other measurements include X-ray emission and ultraviolet absorption lines from metal-enriched gas in galactic halos. Astronomers have identified large reservoirs of baryonic matter in the circumgalactic medium (CGM) and intergalactic medium (IGM) that contain 50-70% of the cosmological baryons formed in the Big Bang. Investigations of physical processes at the “edge of galaxies” help define the importance of this gas in sustaining the star formation in galaxies.