  Combined HST, X-ray, and ground-based optical studies show that clusters of
galaxies are largely "in place" by $z \sim 1$, an epoch when the Universe was
less than half its present age. High resolution images show that elliptical,
S0, and spiral galaxies are present in clusters at redshifts up to $z \sim
1.3$. Analysis of the CMDs suggest that the cluster ellipticals formed their
stars several Gyr earlier, near redshift 3. The morphology--density relation is
well established at $z\sim1$, with star-forming spirals and irregulars residing
mostly in the outer parts of the clusters and E/S0s concentrated in dense
clumps. The intracluster medium has already reached the metallicity of
present-day clusters. The distributions of the hot gas and early-type galaxies
are similar in $z\sim1$ clusters, indicating both have largely virialized in
the deepest potentials wells. In spite of the many similarities between
$z\sim1$ and present-day clusters, there are significant differences. The
morphologies revealed by the hot gas, and particularly the early-type galaxies,
are elongated rather than spherical. We appear to be observing the clusters at
an epoch when the sub-clusters and groups are still assembling into a single
regular cluster. Support for this picture comes from CL0152 where the gas
appears to be lagging behind the luminous and dark mass in two merging
sub-components. Moreover, the luminosity difference between the first and
second brightest cluster galaxies at $z\sim1$ is smaller than in 93% of
present-day Abell clusters, which suggests that considerable luminosity
evolution through merging has occurred since that epoch. Evolution is also seen
in the bolometric X-ray luminosity function.
