  Because H2 formation on dust grain surfaces completely dominates gas-phase H2
formation in local molecular clouds, it is often assumed that gas-phase
formation is never important. In fact, it is the dominant mechanism in a number
of cases. In this paper, I briefly summarize the chemistry of gas-phase H2
formation, and show that it dominates for dust-to-gas ratios less than a
critical value D_cr. I also show that D_cr is simple to calculate for any given
astrophysical situation, and illustrate this with a number of examples, ranging
from H2 formation in warm atomic gas in the Milky Way to the formation of
protogalaxies at high redshift.
