Our understanding of galaxy formation and evolution critically depends
on our ability of exposing the properties of the gaseous content of galaxies
throughout cosmic history: how much gas is there, in which phase (ionized,
atomic, molecular?), in which physical conditions (temperature, density), how
efficiently does it turn into stars? We are now entering an exciting era where these
questions can be addressed via observations of various gas tracers, especially at
mm and sub-mm wavelengths. I will review how to observe various gas phases at
high redshift, and discuss lessons we have learned so far from campaigns aimed
at characterizing the gas content in galaxies in various cosmic epochs
