The main conclusions are as follows:
The only AGN detected at GeV and even TeV energies are blazars, that is a special class of sources which includes BL Lacertae objects and radio quasars with a relatively flat radio spectrum. These sources are thought to have their jets moving at relativistic speeds almost directly towards the observer, a phenomenon which goes under the name of ``relativistic beaming'' and causes strong amplification and collimation of the radiation in our rest frame.
As blazars make up at most 5% of all AGN,
they must have some
peculiar characteristics which favor their 
-ray detection. I have
shown that the presence of high-energy particles, relativistic beaming, and a
strong non-thermal (jet) component play, in fact, a fundamental role in making
these sources detectable at -ray energies.
-ray
missions ~ 1,000 times more sensitive than EGRET might
also detect the bulk of the more common radio-quiet AGN, under the assumption
that they also possess, on much smaller scales, a non-thermal engine.
TeV astronomy, a very young branch of astronomy which has already produced some very exciting results, will likely play an important role in the near future in constraining blazar models. More sensitive TeV telescopes are clearly needed.
In summary, there exists a tight connection between unified schemes and
-ray emission in
AGN, as they both depend on relativistic beaming, the
former as a mechanism to produce a strong angle dependence of the observed
properties, the latter as a way to let -ray photons escape from the
source.