  An introduction is given to a meeting on the role of massive and stellar
black holes in powering non-thermal activity in a rich variety of cosmic
sources. Relevant properties of magnetized, spinning black holes are summarized
and their observational expression, within galactic nuclei, in terms of radio
loudness and Fanaroff-Riley class, is briefly described. The dependence of the
accretion mode on the rate and manner of the mass supply beyond the black hole
sphere of influence is also discussed. It is argued that hydromagnetic outflows
from accretion disks are generally expected over as many as six decades of
radius and that they may be the source of emission line gas. These outflows
collimate the relativistic jets which are probably generated in an
electromagnetic form but become hydromagnetic as they entrain gas through
boundary layers where most of the initial nonthermal emission occurs. It is
proposed that the particle acceleration close to the hole emphasizes the proton
channel which allows secondary pairs to be created at far higher energies than
is possible from direct acceleration. These pairs radiate synchrotron
gamma-rays which can escape along the jet because the outflow effectively
shields them from pair-producing, soft photons. Jets are subject to helical
instabilities which can tangle their magnetic field and may destroy them. The
jet should become plasma-dominated through intermittent, "magnetoluminescent"
untangling of the field which causes nonthermal emission all along its length.
Powerful jets remain supersonic out to the "hot spots" at the extremities of
the source; weaker jets become subsonic plumes or bubbles. The prospects for
learning much more about the nature and operation of jets over the next decade
are excellent.
