  The power of blazar jets rivals the power that gravity can extract from
accreting matter. The mechanism launching and accelerating jets can be
considered as the most efficient engine operating in radio--loud sources. It is
still a matter of debate if the jet carries this power to the radio lobes,
hundreds of kpc away, in the form of Poynting flux or bulk kinetic energy, or
both, and if these two ingredients have relative weights changing along the
way. Accordingly, there are two (or more) possible general scenarios for how
the jet can dissipate part of its power into radiation. It can be through e.g.
reconnection of the magnetic field in the purely electromagnetic scenario, or
through internal shocks in the matter dominated picture. Ways to discriminate
these ideas are welcome.
