The relativistic jet paradigm seems to explain a wide range of characteristics observed in compact jets. This is remarkable given the relative simplicity of both the geometry and physics assumed. Still, one has the impression from both numerical simulations of jet fluid dynamics and observations such as those of M 87 (see Biretta's contribution to these proceedings) that actual jets are much more complex than the basic model allows. I hope that the high dynamic range allowed by the VLBA and future simulations of jets with relativistic flow velocities will soon allow us to inject a larger dose of realism into the models. It is almost certainly true that we will find that some of our simplifying assumptions are woefully inadequate to the task of describing compact jets.
It is somewhat disappointing that the smallest observable features of the radio jets are much larger than the expected interaction scale between the central engine and the deepest regions of the plasma flow that emerges downstream as a well-collimated relativistic jet. Fortunately, it now appears that this "inner jet" region produces a high luminosity in X-rays and rays, and is therefore observationally accessible. Multifrequency monitoring of a few selected objects across all wavebands are now planned for late 1992 and 1993. Inspection of the data for correlated variability and time delays should reveal the relative location of the nonthermal emission at each waveband. This will allow us to infer the geometry and physics of the inner jet, which in turn should lead to a better understanding of the processes by which the jet is focused and accelerated. When combined with observations of the thermal emission from active galaxies, this information should allow us to develop a complete model of active galaxies, from the central engine to the jet and extended lobes.
The author's work on compact jets is supported in part by National Science Foundation grant AST-9116525 and NASA grants NAGW-1068 and NAG 5-1566.