Cosmic evolution of gas content and accretion

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.