  Active Galactic Nuclei can be copious extragalactic emitters of MeV-GeV-TeV
gamma rays, a phenomenon linked to the presence of relativistic jets powered by
a super-massive black hole in the center of the host galaxy. Most of gamma-ray
emitting active galactic nuclei, with more than 1500 known at GeV energies, and
more than 60 at TeV energies, are called "blazars". The standard blazar
paradigm features a jet of relativistic magnetized plasma ejected from the
neighborhood of a spinning and accreting super-massive black hole, close to the
observer direction. Two classes of blazars are distinguished from observations:
the flat-spectrum radio-quasar class (FSRQ) is characterized by strong external
radiation fields, emission of broad optical lines, and dust tori. The BL Lac
class (from the name of one of its members, BL Lacertae) corresponds to weaker
advection-dominated flows with gamma-ray spectra dominated by the inverse
Compton effect on synchrotron photons. This paradigm has been very successful
for modeling the broadband spectral energy distributions of blazars. However,
many fundamental issues remain, including the role of hadronic processes and
the rapid variability of those BL Lac objects whose synchrotron spectrum peaks
at UV or X-ray frequencies. A class of gamma-ray--emitting radio galaxies,
which are thought to be the misaligned counterparts of blazars, has emerged
from the results of the Fermi-Large Area Telescope and of ground-based
Cherenkov telescopes. Blazars and their misaligned ounterparts make up most of
the >100 MeV extragalactic gamma ray background (EGB), and are uspected of
being the sources of ultra-high energy cosmic rays. The future "Cherenkov
Telescope Array", in synergy with the Fermi-Large Area Telescope and a wide
range of telescopes in space and on he ground, will write the next chapter of
blazar physics.
