This paper gives an overview of various excitation and decay processes in atoms, molecules and solids, discussing the physics underlying electron and photon spectra excited with e.g. synchrotron light or electron beams. We shall describe the conditions under which a system will show strong many‐body effects in core level spectra and discuss some explicit examples of collective effects, breakdown of one‐electron pictures for core holes, resonance excitation‐emission and threshold effects.