  Gravitational lensing has become one of the most interesting tools to study
the mass distribution in the Universe. Since gravitational light deflection is
independent of the nature and state of the matter, it is ideally suited to
investigate the distribution of all (and thus also of dark) matter in the
Universe. Lensing results have now become available over a wide range of
scales, from the search for MACHOs in the Galactic halo, to the mass
distribution in galaxies and clusters of galaxies, and the statistical
properties of the large-scale matter distribution in the Universe. Here, after
introducing the concepts of strong and weak lensing, several applications are
outlined, from strong lensing by galaxies, to strong and weak lensing by
clusters and the lensing properties of the large-scale structure.
