  The radiation we observe from blazars is most likely the product of the
transformation of bulk kinetic energy into random energy. This process must
have a relatively small efficiency (e.g. 10%) if jets are to power the extended
radio-structures. Recent results suggest that the average power reaching the
extended radio regions and lobes is of the same order of that produced by
accretion and illuminating the emission line clouds. Most of the radiative
power is produced in a well localized region of the jet, and, at least during
flares, is mainly emitted in the gamma-ray band. A possible scenario
qualitatively accounting for these facts is the internal shock model, in which
the central engine produces a relativistic plasma flow in an intermittent way.
