Abstract: Gravitational waves open the possibility to investigate the nature of compact objects and probe the existence of horizons in black holes. This is of particular interest given some quantum-gravity models which predict the presence of horizonless and singularity-free compact objects. Such exotic compact objects can emit a different gravitational-wave signal relative to the black hole case. In this talk, I will derive the ringdown oscillation frequencies of horizonless compact objects and discuss an analytical template to search for the gravitational-wave echoes emitted in the late-time postmerger signal.