The most powerful and direct probe into the internal mass structure of galaxies is through gravitational lensing. When a distant galaxy is closely lined up with the center of a foreground massive galaxy, the distant one can be gravitationally lensed into several images, or in some cases into a full Einstein Ring. I will talk about the ongoing SDSS-Lens ACS (SLACS) Survey, where spectroscopically-identified strong lens candidates are being followed up with Hubble Space Telescope imaging, and which has found more than 40 spectacular new lenses to date -- fewer than 100 total had been known till now. I will also show preliminary results on the evolution of the internal structure of massive galaxies over several Gyr of cosmic time, and will showcase other galaxy evolution and cosmological studies that this new uniform dataset enables.