Marina De Amicis, a researcher at the Perimeter Institute, presents this talk as part of the Institute for Advanced Study workshop Black Holes from Theory to Observations, held in Wolfensohn Hall. She addresses how physicists model the behavior of a black hole in the moments after two black holes merge, when the resulting object settles into its final stationary state through a ringdown phase governed by perturbation theory and quasinormal modes. The talk connects these theoretical models to the gravitational wave signals detected by observatories like LIGO, discussing what the post-merger waveform can reveal about the properties of the final black hole and where current models succeed or break down. Aimed at an audience already versed in general relativity and gravitational wave physics, the talk runs 43 minutes and fits within a broader workshop on bridging theoretical predictions with observational data from compact object mergers.