In this talk we consider nonlinear Klein-Gordon equations with potential. We prove that spatially localized and time-periodic bound states of the linear problem may be destroyed by generic nonlinear Hamiltonian perturbations, via energy transfers from the discrete to continuum modes and slow radiation of energy to infinity. We explore the underlying mechanism (generalized Fermi's Golden Rule) of such phenomenon and give descriptions on the transfer rate in the full generality: small or large and single or multiple eigenvalues, high dimensional eigenspaces. This settles a long-standing problem raised in the paper of Soffer-Weinstein 1999, in which single and large eigenvalue case was first treated. This is a jiont work with my students Jie Liu and Zhaojie Yang.