  Being dominated by non-thermal emission from aligned relativistic jets,
blazars allow us to elucidate the physics of extragalactic jets, and,
ultimately, how energy is extracted from the central black hole. Crucial
information about jet structure is provided by the spectral energy
distributions from radio to gamma-rays, their trends with luminosity, and
correlated multifrequency variability. Since blazar jets have broad
implications for all radio-loud (and possibly radio-quiet) AGNs, we also need
to understand their circumnuclear structure, especially the details of the
physical and dynamical conditions of the highly ionized gas on sub-pc scales,
which could be directly related to jet formation and radiative power.
Eventually, the bulk of information provided by blazars will help us clarify
the origin of the radio-loud/radio-quiet AGN dichotomy, one of the most
outstanding open issues of extragalactic astrophysics.
