A gravitational-wave (GW) signal emitted from a binary black hole merger is an important probe of gravity. The observation of GW ringdown allows us to perform a robust test of gravity as the ringdown waveform is a superposition of quasi-normal (QN) modes of the remnant black hole. It is known that black holes have an infinite number of QN modes. The precise measurement of multiple QN modes puts Einstein’s general relativity to the test. Also, the tentative detection of anomalous signals following the ringdown signal, i.e., GW echoes, has been claimed. The echo is postulated to be the footprint of quantum gravitational effects that can be significant in the vicinity of black hole horizons.I will explain how we can probe classical and quantum gravity with the observation of black hole ringing and will review the open issues and progress in this field.