The propagation of cosmological ionization fronts during the reionization of the universe is strongly influenced by small-scale ($\sim$ kpc) gas inhomogeneities caused by structure formation. In this paper we study this important effect by performing detailed radiative-hydrodynamic simulations of photoevaporation of cosmological minihalos (MHs) and incorporating the results into a semi-analytical model of reionization, which also includes the effect of mean intergalactic medium (IGM) clumping and the nonlinear clustering of minihalos. We find that small-scale structures have a significant effect on the process of reionization, slowing it down and extending it in time. This can help in understanding the recent observations by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite, which point to an early and extended reionization epoch.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html