  There is now unequivocal evidence that the jets in FR I radio galaxies are
initially relativistic, decelerating flows. On the assumption that they are
axisymmetric and intrinsically symmetrical (a good approximation close to the
nucleus), we can make models of their geometry, velocity, emissivity and field
structure whose parameters can be determined by fitting to deep VLA
observations. Mass entrainment - either from stellar mass loss within the jet
volume or via a boundary layer at the jet surface - is the most likely cause
for deceleration. This idea is quantitatively consistent with the velocity
field and geometry inferred from kinematic modelling and the external gas
density and pressure profiles derived from X-ray observations. The jets must
initially be very light, perhaps with an electron-positron composition.
