  In the present universe, the gas is a minor component of giant galaxies, and
its dominant phase is atomic (HI). During galaxy evolution in cosmic times,
models predict that gas fractions were much higher in galaxies, and gas phases
could be more balanced between molecular (H2) and atomic (HI). This gaseous
evolution is certainly a key factor to explain the cosmic evolution of the star
formation rate density. Star formation efficiency might also vary with
redshift, and the relative importance of these factors is not yet well known.
Our current knowledge of cosmic evolution of gas from molecular observations at
high-z is reviewed and confronted to simulations.
