  In the last years, progress has been very rapid in the domain of molecules at
high redshift, and we know in better detail now the molecular and dust content
in several systems beyond z=1 and up to z = 5.
  The first discovery in 1992 by Brown and van den Bout of CO lines at z=2.28
in a gravitationally lensed starburst galaxy, strongly stimulated searches of
other systems, but these were harder than foreseen, and less than 10 other
systems have been discovered in CO emission. Redshifts range between 2 and 5,
the largest being BR1202-0725 at z=4.69. Most of these systems, if not all, are
gravitationally amplified objects. Some have been discovered first through
their dust emission, relatively easy to detect because of the negative
K-correction effect. The detection of all these systems could give an answer
about the debated question of the star-formation rate as a function of
redshift. The maximum of star-formation rate, found around z=2 from optical
studies, could shift to higher z if the most remote objects are hidden by dust.
  Absorption in front of quasars can also probe cold gas at high redshift,
taking advantage of very high spatial (milli arcsec) and spectral (30m/s)
resolutions. From the diffuse components, one can measure the cosmic black body
temperature as a function of redshift. All these preliminary studies will be
carried out at large scales with future millimeter instruments, and some
perspectives are given.
