  In this paper we discuss models of the X-rays and TeV gamma-ray emission from
BL Lac objects based on parallel electron-positron or electron-proton beams
that form close to the central black hole owing to the strong electric fields
generated by the accretion disk and possibly also by the black hole itself.
Fitting the energy spectrum of the BL Lac object Mrk 501, we obtain tight
constrains on the beam properties. Launching a sufficiently energetic beam
requires rather strong magnetic fields close to the black hole 100-1000 G.
However, the model fits imply that the magnetic field in the emission region is
only 0.02 G. Thus, the particles are accelerated close to the black hole and
propagate a considerable distance before instabilities trigger the dissipation
of energy through synchrotron and self-Compton emission. We discuss various
approaches to generate enough power to drive the jet and, at the same time, to
accelerate particles to 20 TeV energies. Although the parallel beam model has
its own problems, it explains some of the long-standing problems that plague
models based on Fermi type particle acceleration, like the presence of a very
high minimum Lorentz factor of accelerated particles. We conclude with a brief
discussion of the implications of the model for the difference between the
processes of jet formation in BL Lac type objects and in quasars.
