  Constraints on the dynamics, dissipation, and production of VHE neutrinos in
relativistic jets are derived using opacity calculations and VHE $\gamma$-ray
observations. In particular, it is demonstrated how rapid variability of the
$\gamma$-ray emission at very high energies ($> 100$ GeV) can be used to map
the location of the $\gamma$-spheres, to derive lower limits on the Doppler
factor of the $\gamma$-ray emission zone, and to constrain the photopion
production opacity. The apparent discrepancy between jet Lorentz factors
inferred from superluminal motions and source statistics in the TeV blazars and
those derived from the $\gamma$-ray emission is discussed. The relation to the
high-energy emission from the HST1 knot in M87 is briefly mentioned. Estimates
of neutrino yields in upcoming neutrino telescopes are given for various
sources. It is shown that for TeV blazars, the rapid variability of the TeV
emission implies neutrino yields well below detection limit.
