In this thesis we show how to use the asymptotic iteration method (AIM) to numerically calculate the quasinormal modes (QNMs) of different (Schwarzschild, Reissner-Nordström and Kerr) black holes in four-dimensional spacetime. For Schwarzschild black holes, we compute the quasinormal frequencies of the gravitational perturbations. For the Kerr black holes, we consider both the scalar and the gravitational cases. We discuss our results especially for the low-lying modes, and compare them to previously published results.