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In the past few years, a few dark matter subhalos of elliptical galaxies have been found by detecting their perturbations of gravitationally lensed images, and their corresponding masses have been inferred. I will demonstrate that the subhalo mass inferred by this method depends critically on the assumed density profile of the subhalo as well as assumptions about its tidal radius. The reported masses of subhalos detected thus far are likely to have significant systematic errors due to these uncertainties. We have found, however, that one can define a projected mass scale for subhalo lensing perturbations that is robust, and thus will allow for a better comparison of the subhalo mass function obtained from lensing to that of dark matter simulations. I will also discuss the prospects for constraining the density profile of massive perturbing subhalos in addition to their masses, which would provide an important test for alternative (non-CDM) dark matter models.

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