Quasars beyond redshift z ~ 6 (age of the Universe 6 quasars are therefore ideal laboratories to characterize the physical properties of the interstellar medium in extreme conditions, and they provide an insight of the interplay between star formation and black hole accretion in massive galaxies emerging from cosmic dawn. The redshift z ~ 6 range is of particular interest because it coincides with the last phase transition of the Universe — from neutral to mostly ionized. Hence, z > 6 quasars can be used as beacons to shed new light on the properties of intergalactic medium at the end of the Epoch of Reionization. Models and numerical simulations of massive black hole formation at early epochs predict that z>6 quasars reside at the extreme peak of the large-scale density structure such that they can be used to pinpoint the first galaxy overdensities. [abridged]