  The variability of the high-frequency peaked BL Lac object PKS 2155-304 is
studied using almost 10 years of optical, X-ray and gamma-rays data. Publicly
available data have been gathered and analyzed with the aim to characterize the
variability and to search for log-normality or periodic behavior. The optical
and X-ray range follow a log-normal process; a hint for a periodicity of about
700 days is found in optical and in the high energy (100 MeV <E<300 GeV) range.
A one zone, time-dependent, synchrotron self-Compton model is successfully used
to reproduce the evolution with energy of the variability and the tentatively
reported periodicity.
