  Molecular absorption lines measured along the line of sight of distant
quasars are important probes of the gas evolution in galaxies as a function of
redshift.
  A review is made of the handful of molecular absorbing systems studied so
far, with the present sensitivity of mm instruments. They produce information
on the chemistry of the ISM at z \sim 1, the physical state of the gas, in
terms of clumpiness, density and temperature. The CMB temperature can be
derived as a function of z, and also any possible variations of fundamental
constants can be constrained. With the sensitivity of ALMA, many more absorbing
systems can be studied, for which some predictions and perspectives are
described.
