  Most problems in gravitational lensing require numerical solutions. The most
frequent types of problems are (1) finding multiple images of a single source
and classifying the images according to their properties like magnification or
distortion; (2) propagating light rays through large cosmological simulations;
and (3) reconstructing mass distributions from their tidal field. This lecture
describes methods for solving such problems. Emphasis is put on using
adaptive-grid methods for finding images, issues of spatial resolution and
reliability of statistics for weak lensing by large-scale structures, and
methodical questions related to shear-inversion techniques.
