  I show how weak gravitational lensing can be used to image the 3-D mass
distribution in the Universe. An inverse relation to the lensing equation,
relating the lensing potential evaluated at each source to the full 3-D
Newtonian potential, is derived. I consider the normal modes of the lensing
problem and clarify the equations using a small-angle approximation. Finally I
consider the prospects of using this method to estimate the 3-D matter
distribution from a realistic galaxy lensing survey.
