  Massive clusters of galaxies gravitationally shear the images of faint
background galaxies. At large impact parameters the shear is weak, but can
still be measured, to a reasonable degree of significance, as a statistical
anisotropy of the faint galaxy images. We describe techniques for measuring the
shear and discuss the interpretation of the shear field. We have applied this
analysis to ms1224+007. We find a clear detection of the shear, but,
puzzlingly, we find a mass about three times that obtained by application of
the virial theorem, and obtain a very large mass-to-light ratio. Similar
results have been obtained by other groups, and we discuss their implications.
