  We have not identified for sure what is the mechanism launching, accelerating
and collimating relativistic jets. The two most likely possibilities are the
gravitational energy of the accreting matter or the rotational energy of a
spinning black hole. Even the evaluation of the jet power is not trivial, since
the radiation from the jet is enhanced by relativistic beaming, and there are
fundamental uncertainties concerning the matter content of the jet
(electron--proton or electron--positron plasma). However, in recent years,
there have been crucial advances mainly driven by the richness of data in the
$\gamma$-ray band. This is the band where blazars emit most of their
electromagnetic power. Furthermore, there are now large sample of $\gamma$-ray
loud blazars covered by optical spectroscopy. For the blazar sub-class of flat
spectrum radio quasars (FSRQ) these data provide measurements of the main
emission lines and of the underlying continuum. From these data, it is
relatively easy to infer the bolometric luminosity of the accretion disk. The
relativistic jet emission on one hand, and the disk luminosity on the other
hand, allows us to compare the jet power and the accretion luminosity. Although
the inferred jet power is subject to a few assumptions and is somewhat
model--dependent, it is possible to derive a lower limit to the jet power that
is assumption-free and model-independent. Since this lower limit is of the
order of the accretion luminosity, we infer that the true jet power is larger.
