  This paper is intended as an introduction to the theory of weak lensing. A
review of the inversion formula introduced by Kaiser and Squires is presented.
We then prove the formula of the aperture densitometry method in a simple way
that allows a clear understanding of where the various terms come from. This is
particularly useful to quantitatively measure masses in any region of a lens.
We then summarize what has been learned from observations of strong lensing
about the dark matter distribution; weak lensing should provide similar
information on larger scales in clusters of galaxies.
