  Molecular hydrogen is the most abundant molecule in the universe. It is the
first one to form and survive photo-dissociation in tenuous environments. Its
formation involves catalytic reactions on the surface of interstellar grains.
The micro-physics of the formation process has been investigated intensively in
the last 20 years, in parallel of new astrophysical observational and modeling
progresses. In the perspectives of the probable revolution brought by the
future satellite JWST, this article has been written to present what we think
we know about the H$_2$ formation in a variety of interstellar environments.
