  Various technical developments enlarged the potential of angle-resolved photo
emission (ARPES) tremendously during the last one or two decades. In particular
improved momentum and energy resolution as well as the use of photon energies
from few eV up to several keV makes ARPES a rather unique tool to investigate
the electronic properties of solids and surfaces. Obviously, this rises the
need for a corresponding theoretical formalism that allows to accompany
experimental ARPES studies in an adequate way. As will be demonstrated by
several examples this goal could be achieved by various recent developments on
the basis of the one-step model of photo emission: The spin-orbit induced
Rashba-splitting of Shockley-type surface states is discussed using a fully
relativistic description. The impact of chemical disorder within surface layers
can be handled by means of the Coherent Potential Approximation (CPA) alloy
theory. Calculating phonon properties together with the corresponding
electron-phonon self-energy allows a direct comparison with features in the
ARPES spectra caused by electron-phonon interaction. The same holds for the
influence of electronic correlation effects. These are accounted for by means
of the dynamical mean field theory (DMFT) that removes the most serious short
comings of standard calculations based on the standard LDA. The combination of
this approach with the CPA allows the investigation of correlated transition
metal alloys. Finally, accounting for the photon momentum and going beyond the
single scatter approximation for the final state allows to deal quantitatively
with ARPES in the HAXPES regime that reduces the influence of the surface on
the spectra and probing primarily the bulk electronic structure this way.
Corresponding calculations of ARPES spectra, however, have to deal with thermal
vibrations in an adequate way.
