Circumstellar disks surrounding young forming stars, are likely the location where planets form.
While the gaseous phase represents up to ~99% of the disk mass and
control the dynamics,
most of disk properties relies on dust analyses.
The main constituent of the gaseous component, molecular hydrogen (H2), remains nearly out of reach and the gas disk is probed through emission lines of minor tracers, such as CO.
In this lecture, we will first recall how H2 symmetric molecular structure makes its detection difficult. We will then review the most significant results achieved so far, thanks to new generation of ground and space-based telescopes, with a special emphasize given to Herbig Ae/be, which are pre-main sequence stars of intermediate mass. Though the first direct estimates of circumstellar disk mass have been reported, observation of H2 is still challenging detection.