Millimeter astronomy at high redshift

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.