  The evolution of the X-ray luminosity function of clusters of galaxies has
been measured to z=0.9 using over 150 X-ray selected clusters discovered in the
WARPS survey. We find no evidence for evolution of the luminosity function at
any luminosity or redshift. The observations constrain the evolution of the
space density of moderate luminosity clusters to be very small, and much less
than predicted by most models of the growth of structure with Omega_m=1. All
the current X-ray surveys agree on this result. Several notable luminous
clusters at z>0.8 have been found, including one cluster which is more luminous
(and is probably more massive) than the well known MS1054 cluster.
