  Through the modelling of the Spectral Energy Distribution of blazars we can
infer the physical parameters required to originate the flux we see. Then we
can estimate the power of blazar jets in the form of matter and fields. These
estimates are rather robust for all classes of blazars, although they are in
part dependent of the chosen model (i.e. leptonic rather than adronic). The
indication is that, in almost all cases, the carried Poynting flux is not
dominant, while protons should carry most of the power. In emission line
blazars the jet has a comparable, and often larger, power than the luminosity
of the accretion disk. This is even more true for line-less BL Lacs. If the jet
is structured at the sub-pc scale, with a fast spine surrounded by a slower
layer, then one component sees the radiation of the other boosted, and this
interplay enhances the Inverse Compton flux of both. Since the layer emission
is less beamed, it can be seen also at large viewing angles, making
radio-galaxies very interesting GLAST candidates. Such structures need not be
stable components, and can form and disappear rapidly. Ultrafast TeV
variability is challenging all existing models, suggesting that at least parts
of the jets are moving with large bulk Lorentz factors and at extremely small
viewing angles. However, these fast "bullets" are not necessarily challenging
our main ideas about the energetics and the composition of the bulk of the jet.
