  Recent observational and theoretical results on blazars are presented and
discussed. We are beginning to understand the rich phenomenology of blazars,
and we are finding trends which will hopefully lead us to unveil the physics of
these extreme sources. Limits and constraints on how jets originate can be
derived by their intense and variable $\gamma$--ray emission. We are now
starting to study their variability behavior simultaneously at different
frequencies, to find delays and lags, to put constraints on emission models and
source geometry. We observe trends in the overall spectral energy distribution.
Also the problems of the overall energetics of jets and of their matter content
are now being tackled.
