  Measurement of the gravitational distortion of images of distant galaxies is
rapidly becoming established as a powerful probe of the dark mass distribution
in clusters of galaxies. With the advent of large mosaics of CCD's these
methods should provide a composite total mass profile for galaxy haloes and
also measure the power spectrum of mass fluctuations on supercluster scales. We
describe how HST observations have been used to place the observational
measurement of the shear on a quantitative footing. By artifically stretching
and then degrading WFPC2 images to simulate ground based observing it is now
possible to directly calibrate the effect of atmospheric seeing. Similar
experiments show that one can remove the effect of artificial image anisotropy
arising in the atmosphere or telescope. There have also been important advances
in the interpretation of the shear: reconstruction techniques have been
extended to encompass the strong distortion regime of giant arcs etc., progress
has been made in removing a bias present in earlier reconstruction techniques,
and we describe new techniques for `aperture densitometry'. We present some new
results on clusters of galaxies, and discuss the intimate connections between
weak lensing and deep spectroscopy.
