  We briefly review our current knowledge of the space density of distant X-ray
clusters as measured by several ROSAT serendipitous surveys. We compare old and
new determinations of the cluster X-ray Luminosity Function (XLF) at increasing
redshifts, addressing the controversial issue of the evolution of its high end.
We use complete subsamples, drawn from the ROSAT Deep Cluster Survey (RDCS), to
quantify the statistical significance of the XLF evolution out to z ~1. A
consistent observational picture emerges in which the bulk of the cluster
population shows no significant evolution out to z ~1, whereas the most
luminous systems (L_x >~ L* [0.5-2 keV] =~ 5x10^44 erg/s) were indeed rarer, at
least at z >0.5, in keeping with the original findings of the EMSS. We also
report on the recent spectroscopic identification of four clusters in the RDCS
lying beyond z =1, the most distant X-ray clusters known to date, which set an
interesting lower limit on the space density of clusters at z >1.
