  Two related topics are discussed. 1. Accretion onto black holes at low and
high (though not very high) rates is believed to proceed adiabatically ({\em
ie} non-radiatively). It is argued that the liberated energy is carried off by
an outflow, probably involving almost all of the gas that is supplied. Two
dimensional, fluid, accretion disks, in which mass, angular momentum and energy
are transported to the disk surface, are summarized. It is conjectured that
relatively minor changes are needed to describe magnetised disks. By contrast,
the disk surface physics is argued to dictate the character of the outflow. 2.
Ultrarelativistic jets appear to be produced by active galactic nuclei (AGN),
pulsars and $\gamma$-ray bursts (GRB). In all three cases, it is argued that
the power is generated electromagnetically by a magnetic rotator, (in a DC not
AC form), and transported in this manner to the emission site. A model of a
relativistically expanding electromagnetic shell is described and used to
provide a simple model of a GRB in which the $\gamma$-rays are produced by
unstable electrical currents flowing along the rotation axis. The shell drives
a relativistic blast wave into the surrounding medium with a speed that varies
with latitude and whose afterglow emission may exhibit achromatic breaks.
Similar processes may be at work in non-relativistic plerions like the Crab
Nebula and, possibly, AGN jets. The observational implications of these two
classes of model and the prospects for performing instructive, numerical
experiments to elucidate them further are briefly outlined.
