  Since the beginning of the new millennium, more than 100 $z\sim 6$ quasars
have been discovered through several surveys and followed-up with
multi-wavelength observations. These data provided a large amount of
information on the growth of supermassive black holes at the early epochs, the
properties of quasar host galaxies and the joint formation and evolution of
these massive systems. We review the properties of the highest-$z$ quasars
known so far, especially focusing on some of the most recent results obtained
in (sub-)millimeter bands. We discuss key observational challenges and open
issues in theoretical models and highlight possible new strategies to improve
our understanding of the galaxy-black hole formation and evolution in the early
Universe.
