Current simulations of galaxy formation make predictions about how mass will be distributed in galaxy halos. Using the technique of gravitational lensing, it is becoming possible to measure details of mass distributions in galaxies beyond the local Universe and, thus, to compare observations directly to simulations for samples of cosmologically distant objects. I will describe results from two ongoing projects that take advantage of high- resolution space- and ground-based imaging, including Keck laser guide star adaptive optics imaging. The first combines strong and weak gravitational lensing to quantify the relative contributions of the dark and luminous matter in galaxies on scales from 10 kpc to nearly 1 Mpc. The second focuses on detecting the subhalos predicted by simulations, via both direct imaging and gravitational lens modeling. Finally, I will present results showing that the combination of a single well-characterized gravitational lens with the WMAP5 data can provide cosmological constraints comparable to those from WMAP5 plus baryon acoustic oscillations.