Superconductivity occurs when the normal state of a metal becomes instable due to Cooper pair formation. Usually it is assumed that the glue for forming electron pairs is provided by phonons. For example, the BCS theory is based on that assumption. After the discovery of the high-temperature superconducting cuprates it was suggested that other bosonic excitations rather than phonons should lead to Cooper pairing. However, arguments have remained vague and a quantitative theory is still lacking. On the other hand recent experiments on the filled skutterudite PrOs₄Sb₁₂ have clearly demonstrated that intra-atomic crystal field excitations of the 4f² shell provide more than 50% to the binding energy of Cooper pairs. The superconducting transition temperature is low in that case. Moreover, in UPd₂Al₃ intra-atomic 5f excitations of the U ions provide not only an explanation for T_c but also for the observed magnetic resonance below T_c and for the heavy quasiparticles found in that system. Phonons play only a negligible role here.