A fundamental problem in the context of Einstein's equations of general relativity is to understand precisely the dynamical evolution of small perturbations of stationary black hole solutions. It is expected that there is a discrete set of characteristic frequencies that play a dominant role at late time intervals and carry information about the nature of the black hole, much like the normal frequencies of a vibrating string. These frequencies are called quasinormal frequencies and they are closely related to scattering resonances in the study of Schrödinger equations. I will introduce a new method for characterizing and studying quasinormal modes for linear wave equations on asymptotically flat black holes.