The time response of a critical reactor to a perturbation in the cross sections has been analyzed in terms of the Green's function of the system. In this way it is possible to visualize the perturbation as a source of neutron waves that are reflected at the boundaries of the system. The original and reflected waves could, in principle, interfere with each other in producing resonances in response to the system; nevertheless, the damping of the neutron waves is usually so large that no dominant frequencies appear. Any observed resonant behavior of the neutron flux has then to be produced by a resonance in the perturbation.