Probing the horizon of black holes with gravitational waves

Gravitational waves open the possibility to investigate the nature of compact objects and probe the horizons of black holes. Some models of modified gravity predict the presence of horizonless and singularity-free compact objects. Such dark compact objects would emit a gravitational-wave signal which differs from the standard black hole scenario. In this chapter, we overview the phenomenology of dark compact objects by analysing their characteristic frequencies in the ringdown and the emission of gravitational-wave echoes in the postmerger signal. We show that future gravitational-wave detectors will allow us to perform model-independent tests of the black hole paradigm.