  We consider some implications of the rapid X-ray and TeV variability observed
in M87 and the TeV blazars. We outline a model for jet focusing and demonstrate
that modest radiative cooling can lead to recollimation of a relativistic jet
in a nozzle having a very small cross-sectional radius. Such a configuration
can produce rapid variability at large distances from the central engine and
may explain recent observations of the HST-1 knot in M87. Possible applications
of this model to TeV blazars are discussed. We also discuss a scenario for the
very rapid TeV flares observed with HESS and MAGIC in some blazars, that
accommodates the relatively small Doppler factors inferred from radio
observations.
