  In this review, I discuss some highlights of recent research on molecular gas
in galaxies; large-scale CO maps of nearby galaxies are being made, which
extend our knowledge on global properties, radial gradients, and spiral
structure of the molecular ISM. Very high resolution are provided by the
interferometers, that reveal high velocity gradients in galaxy nuclei, and
formation of embedded structures, like bars within bars.
  Observation of the CO and other lines in starburst galaxies have questioned
the H2-to-CO conversion factor. Surveys of dwarfs have shown how the conversion
factor depends on metallicity. The molecular content is not deficient in galaxy
clusters, as is the atomic gas. Galaxy interactions are very effective to
enhance gas concentrations and trigger starbursts. Nuclear disks or rings are
frequently observed, that concentrate the star formation activity. Since the
density of starbursting galaxies is strongly increasing with redshift, the CO
lines are a privileged tool to follow evolution of galaxies and observe the ISM
dynamics at high redshift: due to the high excitation of the molecular gas, the
stronger high-$J$ CO lines are redshifted into the observable band, which
facilitates the detection.
