  We discuss the analytic calculation of quasi-normal modes of various types of
perturbations of black holes both in asymptotically flat and anti-de Sitter
spaces. We obtain asymptotic expressions and also show how corrections can be
calculated perturbatively. We pay special attention to low-frequency modes in
anti-de Sitter space because they govern the hydrodynamic properties of a gauge
theory fluid according to the AdS/CFT correspondence. The latter may have
experimental consequencies for the quark-gluon plasma formed in heavy ion
collisions.
