  The evolution of the X-ray luminosity function of clusters of galaxies has
been measured to z=0.85 using over 150 X-ray selected clusters found 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=1.
