  The cosmic reionization era, which includes formation of the first stars,
galaxies, and AGN, is now one of the most active frontiers of cosmological
research. We review briefly our current understanding of the early structure
formation, and use the ideas about a joint formation of massive black holes
(which power the early QSOs) and their host galaxies to employ high-redshift
QSOs as probes of the early galaxy formation and primordial large-scale
structure. There is a growing evidence for a strong biasing in the formation of
the first luminous sources, which would lead to a clumpy reionization.
Absorption spectroscopy of QSOs at z > 6 indicates the end of the reionization
era at z ~ 6; yet measurements from the WMAP satellite suggest and early
reionization at z ~ 10 - 20. The first generation of massive stars, perhaps
aided by the early mini-quasars, may have reionized the universe at such high
redshifts, but their feedback may have disrupted the subsequent star and galaxy
formation, leading to an extended and perhaps multimodal reionization history
ending by z ~ 6. Observations of gamma-ray bursts from the death events of
these putative Population III stars may provide essential insight into the
primordial structure formation, reionization, early chemical enrichment, and
formation of seed black holes which may grow to become central engines of
luminous quasars.
