Photon splitting in atomic fields

Photon splitting due to vacuum polarization in the electric field of an atom is considered. We survey different theoretical approaches to the description of this nonlinear QED process and several attempts of its experimental observation. We present the results of the lowest-order perturbation theory as well as those obtained within the quasiclassical approximation being exact in the external field strength. The experiment where photon splitting was really observed for the first time is discussed in details. The results of this experiment are compared with recent theoretical estimations