Quasi-Normal Modes (QNM) provide a useful tool for exploring approach to thermal equilibrium in black hole physics and more recently, via the AdS/CFT correspondence also to Quantum Field Theory and possibly QCD. After describing QNM's for black holes I will describe several applications of QNM's, including a viscosity bound and thermalization in strongly coupled plasmas. I will conclude with some comments on possible bounds on relaxation times as well as on the validity of the AdS/CFT correspondence at finite N.
