  The spectra of blazars form a sequence which can be parametrized in term of
their observed bolometric luminosity. At the most powerful extreme of the
sequence we find objects whose jet power can rival the power extracted by
accretion, while at the low power end of the sequence we find TeV and TeV
candidate blazars, whose spectral properties can give information on the
particle acceleration mechanism and can help to measure the IR background. Most
of the emission of blazars is produced at a distance of few hundreds
Schwarzschild radii from the center, and must be a small fraction of the
kinetic power carried by the jet itself: most of the jet energy must be
transported outwards, to power the extended radio structures. The radiation
produced by jets can be the result of internal shocks between shells of plasma
with different bulk Lorentz factors. This mechanism, thought to be at the
origin of the gamma-rays observed in gamma-ray bursts, can work even better in
blazars, explaining the observed main characteristics of these objects. Chandra
very recently discovered large scale X-ray jets both in blazars and in
radio-galaxies, which can be explained by enhanced Compton emission with
respect to a pure synchrotron self Compton model. In fact, besides the local
synchrotron emission, the radiation coming from the sub-pc jet core and the
cosmic background radiation can provide seed photons for the scattering
process, enahancing the large scale jet X-ray emission in radio-galaxies and
blazars, respectively.
