High Energy Atomic Physics This self contained text introduces readers to the field of high energy atomic physics a new regime of photon atom interactions in which the photon energies significantly exceed the atomic or molecular binding energies and which opened up with the recent advent of new synchrotron sources. From a theoretical point of view a small parameter characteristic of the bound system emerged making it possible to perform analytic perturbative calculations that can in turn serve as benchmarks for more powerful numerical computations. The first part of the book introduces readers to the foundations of this new regime and its theoretical treatment. In particular the validity of the small parameter perturbation expansion and of the lowest order approximation is critically reviewed. The following chapters then apply these insights to various atomic processes such as photoionization as a many body problem dominant mechanisms for the production of ions at higher energies Compton scattering and ionization accompanied by creation of e e+ pairs and the photoionization of endohedral atoms (e.g. fullerene). Last but not least the computationally challenging transitions in the electron shell during certain types of nuclear decays are investigated in detail. Introduction. What is this book about Box of tools Perturbation theory Singularities of the amplitudes and of the wave functions The Coulomb field. Nonrelativistic case. The Coulomb field. Relativistic case Photoionization of atoms Ionization and excitation by the photon impact at higher energies Ionization and excitation by the photon impact at higher energies Double photoionization and related processes Photoionization of endohedral atoms Annihilation of positrons with atomic electrons Nuclear transitions and electronic shell.