Some of the larger structures in the universe, galaxy clusters can contain just a few galaxies or they can hold thousands of galaxies. Galaxy clusters controbute to our knowledge of the cosmos in myriad ways. The most obvious is that they contain many examples of galaxies for us to study. But they also show us how galaxies interact, which helps us better understand galaxy formation and evolution. One of the most interesting things we find in massive galaxy clusters is the phenomenon of gravitational lensing, which lets us observe objects that would otherwise be well out of reach of even our most powerful telescopes.