  We present the results of a weak lensing survey of six high-redshift (z >
0.5), X-ray selected clusters of galaxies. We have obtained ultra-deep R-band
images of each cluster with the Keck Telescope, and have measured a weak
lensing signal from each cluster. From the background galaxy ellipticities we
create two-dimensional maps of the surface mass density of each cluster. We
find that the substructure seen in the mass reconstructions typically agree
well with substructure in both the cluster galaxy distributions and X-ray
images of the clusters. We also measure the one-dimensional radial profiles of
the lensing signals and fit these with both isothermal spheres and "universal"
CDM profiles. We find that the more massive clusters are less compact and not
as well fit by isothermal spheres as the less massive clusters, possibly
indicating that they are still in the process of collapse.
