  Rapid variability on a time scale much faster than the light-crossing time of
the central supermassive black hole has been seen in TeV emission from the
blazar PKS 2155-304. The most plausible explanation of this puzzling
observation is that the radiating fluid in the relativistic jet is divided into
a large number of sub-regions which move in random directions with relativistic
speeds. We consider two versions of this "jets in a jet" model. In the first,
the "subjets" model, stationary regions in the mean jet frame emit relativistic
subjets that produce the observed radiation. The variability time scale is
determined by the size of the sub-regions in the mean jet frame. This model,
which is motivated by magnetic reconnection, has great difficulty explaining
the observations in PKS 2155-304. In the alternate "turbulence" model, various
sub-regions move relativistically in random directions and the variability time
scale is determined by the size of these regions in their own comoving frames.
This model fits the data much more comfortably. We consider collisions between
TeV photons emitted from different sub-regions and find that, in both the
subjets and turbulence models, the mean bulk Lorentz factor of the jet needs to
be greater than 25 to avoid the pair catastrophe.
