  Gravitational lensing represents a powerful tool to estimate the cosmological
pa- rameters and the distribution of dark matter. I will describe the main
observable quantities, concentrating on strong lensing, that manifests its
effect through the formation of spectacular events, like multiple quasars,
Einstein rings and arcs in clusters of galaxies. In events where a quasar is
lensed by an intervening galaxy, it is possible to give an estimate of the
Hubble constant H_0, by choosing a mass density model for the lens, thus
allowing to constrain the dark matter content.
