  The bulk of the molecular component in galaxies is made of cold H2, which is
not observed directly, but which abundance is derived from indirect tracers
such as CO emission. The CO to H2 conversion ratio remains uncertain, and may
vary by large factors in special environments with different excitation or
metallicity. Recent cold gas discoveries (through gamma-rays or cold dust
emission) are reviewed and the most promising tracers in the future are
discussed, such as the primordial molecules HD and LiH, or the pure rotational
lines of excited H2*.
