Galaxies, in general, do not live in isolation. It has long been observed that galaxies living in environments densely populated with other galaxies form fewer stars, have different shapes, and contain less cold gas than those more isolated. Using triply ionized carbon (C IV), a tracer of ionized halo gas, we find that galaxies in dense environments also show lower incidences of C IV in their CGM -- and in less dense environments than where the deficiency of cold gas and star formation are typically observed. This may indicate that the same processes that eventually shut off star formation and render galaxies devoid of cold gas in their central regions are first evident in the CGM and set in at relatively low densities, much lower than that of galaxy clusters, the densest regions in the Universe. In clusters, I have shown that even neutral hydrogen (H I), which is ubiquitous in more isolated galaxies, is greatly suppressed. In fact, the H I contents of galaxies show an apparent steady decline from the field to groups to clusters, indicating progressively greater environmental influence on galaxies' gas reservoirs.