  The study of molecular gas in quasar host galaxies addresses a number of
interesting questions pertaining to the hosts' ISM, to unified schemes relating
quasars and IR galaxies, and to the processes fueling nuclear activity. In this
contribution I review observations of molecular gas in quasar hosts from z=0.06
to z=4.7. The Cloverleaf quasar at z=2.5 is featured as a case where there are
now enough detected transitions (four in CO, and one each in CI and HCN) to
allow detailed modeling of physical conditions in the molecular ISM. We find
that the CO-emitting gas is warmer, denser, and less optically thick than that
found in typical Galactic molecular clouds. These differences are probably due
to the presence of the luminous quasar in the nucleus of the Cloverleaf's host
galaxy.
