Bright Active Galactic Nuclei are powered by accretion of mass onto the
super massive black holes at the centers of the host galaxies.
For fainter objects star formation may significantly contribute to the luminosity.
We summarize
experimental indicators of the accretion processes in Active Galactic Nuclei (AGN), i.e.,
observable activity indicators that allow us to conclude on the nature of accretion.
The Galactic Center is the closest galactic nucleus that can be studied with unprecedented
angular resolution and sensitivity.
Therefore, here we also include the presentation of recent observational results on Sagittarius~A*
and the conditions for star formation in the central stellar cluster.
We cover results across the electromagnetic spectrum
and find that the Sagittarius~A* (SgrA*) system
is well ordered with respect to its geometrical orientation and its
emission processes of which we assume to reflect the
accretion process onto the super massive black hole.