| Abstract: Throughout cosmic history, the bulk of the hydrogen and metals in the universe has resided outside galaxies in diffuse gaseous structures. Understanding this circumgalactic and intergalactic medium is essential for building a complete picture of galaxy formation and evolution, especially those gaseous structures that are at the interfaces of galaxies and IGM, regulating gas accretion, feedback, and chemical enrichment. Quasar absorption line spectroscopy provides one of the most sensitive methods to study such low-density gas over a wide range of redshifts and physical conditions. Over the past three decades, studies have revealed that the CGM is highly multiphased and that a significant portion of the baryonic content of galaxies resides in the CGM. Models combined with systematic galaxy surveys around quasar sightlines have shown that many metal line absorbers arise in extended halos, tracing inflowing, outflowing, and shock-heated gas, providing direct constraints on the baryon cycle in galaxies predicted by theoretical models. I will summarize key results from quasar absorption line studies and discuss how recent progress, including the use of integral field spectroscopy to identify absorber environments and comparisons with cosmological simulations, is helping to reveal the nature of gas at the interface between galaxies and the intergalactic medium across cosmic time.
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