The discovery of gravitational waves from a binary black hole merger, detected by LIGO and Virgo, has launched a new era of black hole physics. The last phase of such intriguing coalescing phenomena is called "ringdown" and is characterized by the final black hole which settles to its equilibrium configuration via the emission of gravitational waves. The latter are described by the quasi normal modes, damped oscillation modes that represent a gravitational fingerprint of the black hole response to perturbations. In this talk we will see why quasi normal modes are important to test general relativity and what they can tell us about fascinating theories beyond Einstein's gravity.