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Jets are ubiquitous. As astronomical instrumentation has improved we have been able to detect jets over an absolutely phenomenal range of distances and powers. Extremely rapid flows in extragalactic sources explain extraordinarily fast variability, apparent superluminal motions, and the spectra of blazars over 17 decades in frequency. Our viewing of these jets from different angles can explain essentially all of the differences between FR II radio galaxies and radio-loud QSOs as well as the differences between FR I radio galaxies and blazars.

No specific model for the production of cosmic jets is absolutely compelling. While it is highly likely that MHD processes are of importance in the launching and initial collimation of jets, the details of these processes remain extremely controversial. Given the complexity of MHD in full general relativity, this is not surprising. Advances in numerical techniques and computing power are finally allowing tentative explorations of 3-D relativistic MHD flows. However, only when several groups, using different codes and wide ranges of plausible initial conditions, all produce very similar outcomes, will it be fair to claim that the source of jets in AGN is understood.

More confidence can be given to the results on propagating jets, since the results of simulations can be reasonably matched to observations. The idea that deceleration through interaction with the external medium converts FR II type jets into FR I types appears to be valid. Nearly all modern 3-D simulations, whether HD or MHD, whether relativistic or not, tend to produce flows that can propagate over very long distances, though instabilities can yield a ``flailing about'' of the outer portions of the jet. The interaction of the jet with the external medium always provides a sheath of matter that can assist in confining the jet and appears to be able to slow the growth of unstable modes.

Although we have not had the space to discuss them, we must end by recalling that much smaller and weaker, but still relativistic, outflows that can be legitimately characterized as jets are found in some compact binary systems (Mirabel & Rodríguez 1999). Furthermore, reasonably collimated, albeit much slower, flows are common around young stellar objects (YSOs; for a recent review see Richer et al. 2000). The argument that all of these collimated outflows are formed in very similar ways (e.g., Livio 1999) is intriguing, but by no means convincing. The differences in physical conditions are so immense, and our understanding of the origin of extragalactic jets so tenuous, that any claims along these lines are most speculative. It is at least as likely that a substantially different physical mechanism, such as the X-wind model (Shu et al. 2000) dominates the slow, weak jets in YSOs and differentiates them from cosmic radio jets.

This work was supported in part by NASA grant NAG 5-3098, by Research Proposal Enhancement funds at Georgia State University and by the Council on Science and Technology at Princeton University.

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