  Multi-frequency observations are a powerful tool of astrophysical
investigation. Not only is data in each wavelength band providing different
clues to the objects nature, but taken simultaneously, these data can reveal
the mechanisms at work in astrophysical objects. In the past years, joint
multi-frequency observations with the H.E.S.S. telescopes in the very high
energy (VHE, E>100GeV) band and several other experiments in the radio,
optical, X-ray, and high energy (HE, E>100MeV) bands have lead to intriguing
results that will ultimately help answering the open questions of the location
of the very high energy emission, details of the acceleration mechanism, and
the role of the central black hole.
