  Gravitational Lensing is a UNIQUE tool to constrain the mass distribution of
collapsed structures, this is particularly true for galaxies, either on a case
by case basis using multiple images of background sources (such as quasars), or
statistically using the so called galaxy-galaxy lensing technique. First, I
will present the lensing theory, and then discuss the various methods applied
to current observations. Finally, I will review the bright future prospects of
galaxy lensing that will benefit of the development of high resolution, large,
wide and deep (lensing) surveys.
