  We review observations and gravitational lensing theory related to the
magnification of background QSOs by intervening overdensities, and the induced
cross-correlation between sources and foreground galaxies. We pay special
attention to simulations, and present some preliminary results from high
resolution cluster simulations, which show the role of the halo core and
substructure on non-linear magnification. For massive clusters, deviations from
the weak gravitational lensing regime are significant on arcmin scales and
bellow. The accumulated knowledge in the field already shows that gravitational
lensing magnification is an important astrophysical and cosmological tool.
