  We review the main results obtained in the literature on quasi-normal modes
of compact stars and black holes, in the light of recent exciting developments
of gravitational wave detectors. Quasi-normal modes are a fundamental feature
of the gravitational signal emitted by compact objects in many astrophysical
processes; we will show that their eigenfrequencies encode interesting
information on the nature and on the inner structure of the emitting source and
we will discuss whether we are ready for a gravitational wave asteroseismology.
