This chapter presents a review of the current state of knowledge on the cool ( T \sim 10 ^ { 4 } K ) halo gas content around massive galaxies at z \approx 0.2 - 2 .
Over the last decade , significant progress has been made in characterizing the cool circumgalactic gas in massive halos of M _ { h } \approx 10 ^ { 12 - 14 } \mbox { $ { M } _ { \odot } $ } at intermediate redshifts using absorption spectroscopy .
Systematic studies of halo gas around massive galaxies beyond the nearby universe are made possible by large spectroscopic samples of galaxies and quasars in public archives .
In addition to accurate and precise constraints for the incidence of cool gas in massive halos , detailed characterizations of gas kinematics and chemical compositions around massive quiescent galaxies at z \approx 0.5 have also been obtained .
Combining all available measurements shows that infalling clouds from external sources are likely the primary source of cool gas detected at d\ > ^ { > } _ { \sim } \ > 100 kpc from massive quiescent galaxies .
The origin of the gas closer in is currently less certain , but SNe Ia driven winds appear to contribute significantly to cool gas found at d < 100 kpc .
In contrast , cool gas observed at d\ > ^ { < } _ { \sim } \ > 200 kpc from luminous quasars appears to be intimately connected to quasar activities on parsec scales .
The observed strong correlation between cool gas covering fraction in quasar host halos and quasar bolometric luminosity remains a puzzle .
Combining absorption-line studies with spatially-resolved emission measurements of both gas and galaxies is the necessary next step to address remaining questions .