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 .