  Multi-wavelength observations with TeV photons are an essential diagnostic
tool to study the physics of TeV sources. The complex variability of blazars,
however (timescales from years down to minutes, with different patterns and SED
behaviours), requires a great effort on simultaneous campaigns, which should be
performed possibly over several days. Spectral information is essential, and
now with the new TeV and X-ray telescopes it can be obtained on timescales less
than one hour. The insights from such observations can be tremendous, since
recent results have shown that the X-ray and TeV emissions do not always follow
the same behaviour, and flares can have different relations between rise and
decay times. Unfortunately, the strong pointing constraints of XMM do not allow
the full use of this satellite simultaneously with ground telescopes.
