  Investigating X-ray luminous galaxy clusters at z>~1 provides a fundamental
constraint on evolutionary studies of the largest virialized structures in the
Universe, the baryonic matter in form of the hot ICM, their galaxy populations,
and the effects of Dark Energy. The main aim of this work is to establish the
observational foundation for the XMM-Newton Distant Cluster Project (XDCP).
This new serendipitous survey is focused on the most distant systems at z>1,
based on the selection of extended X-ray sources, their identification as
clusters via two-band imaging, and their final spectroscopic confirmation.
Almost 1000 extended sources were selected as cluster candidates from the
analysis of 80 deg^2 of deep XMM-Newton archival data, of which 75% could be
readily identified as systems at z<~0.6. For the remaining 250 distant cluster
candidates a new strategy for their confirmation and redshift estimates was
adopted, based on Z- and H-band photometry and the observed Z-H red-sequence
color of early-type cluster galaxies. From observations of 25% of the sample,
more than 20 X-ray clusters were discovered at a photometric redshift of
z>~0.9. The new Z-H method has allowed a cluster sample study over an
unprecedented redshift baseline of 0.2<~z<~1.5. From a comparison of the
observed color evolution of the red-sequence with model predictions, the
formation epoch of early-type galaxies could be constrained as z_f=4.2+-1.1,
confirming their well-established old age. The preliminary investigation of the
H-band luminosity evolution of 63 BCGs provides for the first time direct
observational indications that the most massive cluster galaxies have doubled
their stellar mass since z~1.5. The finding that BCGs were assembled in the
last 9Gyr is now in qualitative agreement with the latest simulations.
