  Recently, there has been a great deal of interest in understanding the
reionization of hydrogen in the intergalactic medium (IGM). One of the major
outstanding questions is how this event proceeds on large scales. Motivated by
numerical simulations, we develop a model for the growth of HII regions during
the reionization era. We associate ionized regions with large-scale density
fluctuations and use the excursion set formalism to model the resulting size
distribution. We then consider ways in which to characterize the morphology of
ionized regions. We show how to construct the power spectrum of fluctuations in
the neutral hydrogen field. The power spectrum contains definite features from
the HII regions that should be observable with the next generation of
low-frequency radio telescopes through surveys of redshifted 21 cm emission
from the reionization era. Finally, we also consider statistical descriptions
beyond the power spectrum and show that our model of reionization qualitatively
changes the distribution of neutral gas in the IGM.
