Large galaxies and galaxy clusters sometimes act like lenses. Their gravity distorts the structure of spacetime, magnifying light from more distant objects. This effect, known as strong gravitational lensing, allows astronomers to study galaxies that would ordinarily be too far to see, map the distribution of mass in the galaxies doing the lensing, and measure the expansion rate of the universe. The CASTLeS (CfA-Arizona Space Telescope Lens Survey) is a program jointly managed by the Center for Astrophysics | Harvard & Smithsonian and the University of Arizona, which used NASA’s Hubble Space Telescope to study multiple aspects of strong gravitational lensing as caused by galaxies. Today, CASTLeS team members maintain a catalog of these lenses, updated with new observational data.