2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/495We review the relevance of H2 molecules for structure formation in cosmology. Molecules are important at high redshifts, when the first collapsed structures appear with typical temperatures of a few hundred Kelvin. In these chemically pristine clouds, radiative cooling is dominated H2 molecules. As a result, H2 ``astro-chemistry'' is likely to determine the epoch when the first astrophysical objects appear. We summarize results of recent three-dimensional simulations. A discussion of the effects of feedback, and implications for the reionization of the universe is also given.9 pages, Invited Review in "H2 in Space", Paris, France, 28 September-1st October 1999, ed. F. Combes and Guillaume Pineau des ForetsAstrophysicstext