The abundance of low mass halos is one of the key predictions of LCDM, which remains at apparent odds with observations of luminous structure. We present new measurements of the spatial distribution and the cumulative luminosity function of satellite galaxies up to a thousand times fainter than their hosts, as a function of host stellar mass and morphology between redshifts 0.1 and 0.8, using imaging from the GOODS and COSMOS fields in conjunction with a rigorous statistical analysis. We demonstrate how these measurements provide powerful new constraints for abundance matching and cosmological simulations in the context of both warm and cold dark matter, and how future measurements of faint satellite colors using CANDELS, will provide important distinguishing power between warm and cold dark matter models. In addition, we present results from a complementary gravitational lens modeling project in which we use strongly lensed AGN narrow-line emission in order to detect dark matter subhalos, demonstrating a promising new method for measuring the subhalo mass function in thousands of lensed systems which will be discovered in ongoing and future optical surveys.