  It becomes possible now to detect cold molecules at high redshift in the
millimeter domain. Since the first discovery in 1992 by Brown and van den Bout
of CO lines at z=2.28 in a gravitationally lensed starburst galaxy, nearly ten
objects are now known to possess large quantities of molecular gas beyond z=1
and up to z = 5, through millimeter and sub-millimeter emission lines.
  The continuum dust emission is the most easily detected: in the mm domain,
the emission is stronger for the more redshifted objects.
  For the CO lines, the situation is less favorable, and the reported
detections are helped by gravitational amplification. The increase of the CMB
temperature T_{bg} with redshift helps the rotational line excitation
(especially at high z), but not its detection.
  Absorption in front of quasars is a more sensitive probe of cold gas at high
redshift, able to detect individual clouds of a few solar masses (instead of
10^{10} Mo for emission). From the diffuse components, one can measure the
cosmic black body temperature as a function of redshift. The high column
densities component allow to observe important molecules not observable from
the ground, like O2, H2O and LiH for example.
