The development of primordial inhomogeneities into the non-linear regime andthe formation of the first bound objects mark the transition from a simplecooling universe -- described by just a few parameters -- to a very messy hotone -- the realm of radiative, hydrodynamic and star formation processes. Inpopular cold dark matter cosmological scenarios, stars and quasars may havefirst appeared in significant numbers around a redshift of 10 or so, as the gaswithin subgalactic halos cooled rapidly due to atomic processes and fragmented.This early generation of sources generated the ultraviolet radiation andkinetic energy that ended the cosmic ``dark ages'' and reheated and reionizedmost of the hydrogen in the cosmos by a redshift of 6. The detailed thermalhistory of the intergalactic medium -- the main repository of baryons at highredshift -- during and soon after these crucial formative stages depends on thepower-spectrum of density fluctuations on small scales and on a complex networkof poorly understood `feedback' mechanisms, and is one of the missing link ingalaxy formation and evolution studies.
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