  This course presents some applications of gravitational lensing to the
measurement of masses of galaxies (galaxy-galaxy lensing, Eintein rings,
perturbations of giant arcs) and cluster of galaxies (strong and weak lensing).
This complements the F. Bernardeau's course dedicated to some theoretical
aspects and weak lensing by large scale structures. In the first section, I
describe the most important lensing quantities and lensing properties useful
for astrophycal applications. Then I briefly present some academic exemples.
Section three is devoted to exemples of mass reconstruction and the study of
mass distribution in clusters of galaxies and galaxies.
