  Gamma-ray emission from AGN provides us with unprecedented insights into the
physics of extragalactic jets. The emission from these jets fits naturally into
the existing theoretical framework of relativistic jets as inferred from parsec
scale and kiloparsec scale observations. Models of the $\gamma$-ray fluxes give
us important knowledge of jet parameters, in particular magnetic fields and the
size of the emitting region. For example, in the case of Markarian~501, the jet
plasma is subequipartiion and the energy flux is particle dominated. Discussion
of the physics of jets on the sub-parsec scale leads naturally to questions of
jet composition. The dynamics of jets are consistent with conventional
electron-proton composition if the minimum Lorentz factors, $\gamma_{\rm min} >
100$. Jets can be electron-positron if $\gamma_{\rm min} \approx 100$.
