  We present a new non-parametric method for determining mean 3D density and
mass profiles from weak lensing measurements around stacked samples of galaxies
or clusters, that is, from measurement of the galaxy-shear or cluster-shear
correlation functions. Since the correlation function is statistically
isotropic, this method evades problems, such as projection of large-scale
structure along the line of sight or halo asphericity, that complicate attempts
to infer masses from weak lensing measurements of individual objects. We
demonstrate the utility of this method in measuring halo profiles, galaxy-mass
and cluster-mass cross-correlation functions, and cluster virial masses. We
test this method on an N-body simulation and show that it correctly and
accurately recovers the 3D density and mass profiles of halos. We find no
evidence of problems due to a mass sheet degeneracy in the simulation.
Cross-correlation lensing provides a powerful method for calibrating the
mass-observable relation for use in measurement of the cluster mass function in
large surveys. It can also be used on large scales to measure and remove the
halo bias and thereby provide a direct measurement of Omega_m * sigma_8.
