  We discuss currently available observational constraints on the reionization
history of the intergalactic medium (IGM), and the extent to which accreting
black holes (BHs) can help explain these observations. We show new evidence,
based on the combined statistics of Lyman alpha and beta absorption in quasar
spectra, that the IGM contains a significant amount of neutral hydrogen, and is
experiencing rapid ionization around redshift z=6. However, we argue that
quasar BHs, even faint ones that are below the detection thresholds of existing
optical surveys, are unlikely to drive the evolution of the neutral fraction
around this epoch, because they would over-produce the present-day soft X-ray
background. On the other hand, the seeds of the quasar BHs around z=6 likely
appeared at much earlier epochs (around z=20), and produced hard ionizing
radiation by accretion. These early BHs are promising candidates to account for
the high redshift ionization (around z=15) implied by the recent cosmic
microwave anisotropy data from WMAP. Using a model for the growth of BHs by
accretion and mergers in a hierarchical cosmology, we suggest that the early
growth of quasars must include a super-Eddington growth phase, and that,
although not yet optically identified, the FIRST radio survey may have already
detected several thousand BHs at z>6 with Mbh>10^8 Msun.
