  We study the resonance phenomena for time periodic perturbations of a
Hamiltonian $H$ on the Hilbert space $L^2(\mathbb R ^d)$. Here, resonances are
characterized in terms of time behavior of the survival probability. Our
approach uses the Floquet-Howland formalism combined with the results of L.
Cattaneo, J.M. Graf and W. Hunziker on resonances for time independent
perturbations.
