  Absorption signatures in the spectra of QSOs are one of our most powerful
tools for studying galactic and intergalactic environments at high redshifts.
With the discovery of QSOs out to z > 7, QSO absorption lines are now tracing
the end stages of reionization on multiple fronts using the hydrogen
Lyman-$\alpha$ forest and heavy element absorbers. Next-generation QSO
absorption line studies with large optical/IR telescopes will reveal in detail
how the first galaxies emerged form the cosmic web, transformed their circum-
and inter-galactic environments, and completed the last major phase transition
of the Universe. These efforts will complement other upcoming studies of
reionization, such as those with JWST, ALMA, and redshifted 21cm experiments.
