Absorption line studies have shown that the circumgalactic medium (CGM) is an extended complex multiphase gas reservoir of galaxies. It is a kinematically diverse region that interfaces the baryon cycle activity within galaxies to the intergalactic environment in which the galaxies are embedded. In this chapter, selected observational programs and their reported results are presented. The focus is on empirical bivariate relations, such as absorption strength and covering fractions, versus impact parameter, stellar mass, star formation rate, etc. The CGM is presented as viewed through several commonly targeted ions, in particular HI, MgII, CIV, OVI, and NeVIII. Though this allows the various ionization stages of CGM gas to be examined in isolation, it glosses over the multiphase nature of the CGM. The practical design of high-redshift experiments is such that they are much more statistical in nature than the more granular experiments at low redshift. Thus, high-redshift studies are discussed separately.