  Most of energy carried by relativistic AGN jets remains undetected until
hundreds of kiloparsecs where interaction with intergalactic medium produces
hot spots. The jet's hidden energy is only partially dissipated at smaller
scales, from parsecs to kiloparsecs. Several media may play the role of the
``prime mover'': ions, pairs or large scale magnetic fields. Analyzing VLBI
polarization structures of relativistic parsec scale jets we conclude that
large-scale magnetic fields can explain the salient polarization properties of
parsec-scale AGN jets. This implies that large-scale magnetic fields carry a
non-negligible fraction of jet luminosity. We also discuss the possibility that
relativistic AGN jets may be electromagnetically (Poynting flux) dominated. In
this case, dissipation of the toroidal magnetic field (and not fluid shocks)
may be responsible for particle acceleration.
