  Clusters of galaxies at high redshift (z>1) are vitally important to
understand the evolution of the large scale structure of the Universe, the
processes shaping galaxy populations and the cycle of the cosmic baryons, and
to constrain cosmological parameters. After 13 years of operation of the
Chandra and XMM-Newton satellites, the discovery and characterization of
distant X-ray clusters is proceeding at a slow pace, due to the low solid angle
covered so far, and the time-expensive observations needed to physically
characterize their intracluster medium (ICM). At present, we know that at z>1
many massive clusters are fully virialized, their ICM is already enriched with
metals, strong cool cores are already in place, and significant star formation
is ongoing in their most massive galaxies, at least at z>1.4. Clearly, the
assembly of a large and well characterized sample of high-z X-ray clusters is a
major goal for the future. We argue that the only means to achieve this is a
survey-optimized X-ray mission capable of offering large solid angle, high
sensitivity, good spectral coverage, low background and angular resolution as
good as 5 arcsec.
