The interplay between interstellar H2 and dust, and observational consequences

In this talk, I will discuss how our understanding of interstellar H2 is affected by the level of details at which dust surface processes are modeled, and the numerous observational impacts it can have : on the interpretation of excited CO Herschel observations in galactic PDRs, on the chemical determination of the cosmic ray flux in the central molecular zone of our galaxy, and on our understanding of the ISO and Spitzer observations of out-of-equilibrium ortho-para ratios of H2 in PDRs.