  Weak gravitational lensing has become an important tool to study the
properties of dark matter halos around galaxies, thanks to the advent of large
panoramic cameras on 4m class telescopes. This area of research has been
developing rapidly in the past few years, and in these proceedings we present
some results based on the Red-Sequence Cluster Survey, thus highlighting what
can be achieved with current data sets. We present results on the measurement
of virial masses as a function of luminosity and the extent of dark matter
halos. Much larger surveys are underway or planned, which will result in an
impressive improvement in the accuracy of the measurements. However, the
interpretation of future results will rely more and more on comparison with
numerical simulations, thus providing direct tests of galaxy formation models.
