2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/10132This 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.6 pages, uuencoded postcript fileAstrophysicstext