  Astrophysical models for the high-energy emission of blazars are reviewed.
Blazars ejecting relativistic radio jets at small angles to the line-of-sight
are the only type of active galactic nuclei (AGN) discovered above 100 MeV. The
gamma-rays apparently originate in the jets which explains the absence of
gamma-ray pair creation attenuation. The bulk Lorentz factor of the radio jets
is much smaller than the Lorentz factors of the emitting particles requiring in
situ particle acceleration such as Fermi acceleration at shocks. The mounting
evidence for a correlation between the optical and gamma-ray emission argues
for the same accelerated electrons emitting the polarized optical synchrotron
photons to be responsible for the high-energy emission. Alternatively, the
gamma-rays could be due to electrons of a secondary origin related to the
energy losses of protons accelerated at the same shocks as the synchrotron
emitting electrons. In this case blazars would produce an observable flux of
high-energy neutrinos. Unified schemes for AGN predict a circum-nuclear warm
dust torus attenuating gamma-rays above ~300 GeV emitted from within a central
sphere of radius ~2 10^4 Schwarzschild radii which rules out external Compton
scattering models as the origin of the TeV gamma-rays from Mrk421 and Mrk501.
