Studies of the highest redshift quasars probe the relation between
the formation of the earliest supermassive black holes and galaxies.
The quasar absorption lines trace the evolution of the intergalactic
medium, and reveal when and how the reionization process happened,
ending the cosmic dark ages. Detections of quasars at z>6 is challenging
the standard picture of black hole growth in early Universe.
They also show strong signs of on-going star-formation from their dust
and gas emissions, suggesting the co-evolution of first generation
black holes and galaxies. The absorption spectra of the highest-redshift
quasars show strong evolution of Ly alpha optical depth, indicating
that the neutral fraction of the intergalactic medium begins to increase
rapidly, and suggests that we might be reaching the tail end of
the reionization process.