The existence of bright QSOs at z>6 shows that black holes as massive
as a few billions of Suns were already in place when the Universe was
less than 1 Gyr old. Theory struggles to explain how these extreme
systems have formed and grown, and is making several - yet untested -
predictions since 15 years.
I will report on some recent observational programs designed at
studying the physics, environment and cosmological evolution of AGN at
very high redshift. These include: i) ALMA observations of QSO hosts
at z~3-6, ii) observations of large-scale galaxy overdensities around
z~6 QSOs; iii) measurements of the luminosity function of z>3 AGN
based on deep X-ray fields such as the CDFS and COSMOS.
Prospects will be finally presented for expanding these programs and
for future mission developments, that may eventually constrain models
of early black hole formation.
