The shear distortion of galaxies behind a large cluster provides us with only a relative measure of the mass of the cluster (Tyson et al. 1990, Kaiser & Squires 1993) due the invariance of shear to a sheet of matter. Thus we can at best only place lower limits on cluster masses, with the constraint that the surface density, κ(θ)=Σ(θ)/Σc, is non-negative. However, an absolute measurement of the mass can be obtained via the magnification of background galaxies (Broadhurst, Taylor & Peacock 1995, hereafter BTP). Here we describe the magnification effect, its observational consequences and mass reconstruction in the linear and nonlinear regimes. We also describe a number of applications in progress.