The recent detections of gravitational waves have revealed an invisible side of the universe: black holes in binaries. These observations test our understanding of black holes, their violent mergers, and the theory of general relativity. A combination of analytic approximations and full numerical simulations is required to understand black hole binaries and to predict the gravitational waves they emit. I will discuss the "ringdown" of the final merged black hole, and present the most recent results from the Advanced LIGO and Virgo detectors.