  I review recent progress in the theory of relativistic jet production, with
special emphasis on unifying black hole sources of stellar and supermassive
size. Observations of both classes of objects, as well as theoretical
considerations, indicate that such jets may be launched with a spine/sheath
flow structure, having a much higher Lorentz factor ($\sim 50$) near the axis
and a lower speed ($\Gamma \sim 10$ or so) away from the axis. It has become
clear that one can no longer consider models of accretion flows without also
considering the production of a jet by that flow. Furthermore, the rotation
rate of the black hole also must be taken into account. It provides a third
parameter that should break the mass/accretion rate degeneracy and perhaps
explain why some sources are radio loud and some radio quiet. Slow jet
acceleration and collimation is expected theoretically, and can explain some of
the observed features of AGN jet sources. Finally, relativistic jets launched
by MHD/ED processes are Poynting flux dominated by nature, and are potentially
unstable if there is significant entrainment of thermal material.
