  Flux distribution is an important tool to understand the variability
processes in active galactic nuclei. We now have available a great deal of
observational evidences pointing towards the presence of log-normal components
in the high energy light curves, and different models have been proposed to
explain these data. Here, we collect some of the recent developments on this
topic using the well-known blazar Mrk 501 as example of complex and interesting
aspects coming from its flux distribution in different energy ranges and at
different timescales. The observational data we refer to are those collected in
a complementary manner by Fermi-LAT over multiple years, and by the First G-APD
Cherenkov Telescope (FACT) telescope and the H.E.S.S. array in correspondence
of the bright flare of June 2014.
