  Multiwavelength (MWL) observations of the blazar PKS 2155-304 during two
weeks in July and August 2006, the period when two exceptional flares at very
high energies (VHE, E>= 100 GeV) occurred, provide a detailed picture of the
evolution of its emission. The complete data set from this campaign is
presented, including observations in VHE gamma-rays (H.E.S.S.), X-rays (RXTE,
CHANDRA, SWIFT XRT), optical (SWIFT UVOT, Bronberg, Watcher, ROTSE), and in the
radio band (NRT, HartRAO, ATCA). Optical and radio light curves from 2004 to
2008 are compared to the available VHE data from this period, to put the 2006
campaign into the context of the long-term evolution of the source. The X-ray
and VHE gamma-ray emission are correlated during the observed high state of the
source, but show no direct connection with longer wavelengths. The long-term
flux evolution in the optical and radio bands is found to be correlated and
shows that the source reaches a high state at long wavelengths after the
occurrence of the VHE flares. Spectral hardening is seen in the SWIFT XRT data.
The nightly averaged high-energy spectra of the non-flaring nights can be
reproduced by a stationary one-zone SSC model, with only small variations in
the parameters. The spectral and flux evolution in the high-energy band during
the night of the second VHE flare is modelled with multi-zone SSC models, which
can provide relatively simple interpretations for the hour time-scale evolution
of the high-energy emission, even for such a complex data set. For the first
time in this type of source, a clear indication is found for a relation between
high activity at high energies and a long-term increase in the low frequency
fluxes.

High Energy Astrophysical Phenomena