  We summarise the recent progress of a project to determine physical
conditions in the jets of low-luminosity (FR I) radio galaxies. We model the
jets as intrinsically symmetrical, relativistic, decelerating flows. By fitting
to deep, high-resolution VLA images in total intensity and linear polarization,
we have been able to derive for the first time the three-dimensional
distributions of velocity, rest-frame emissivity and magnetic-field structure.
We describe our results for four sources and outline the implications for
theories of jet deceleration. We also summarise a conservation-law analysis of
3C 31, which gives profiles of internal pressure, density, Mach number and
entrainment rate along the jets, again for the first time. Finally, we discuss
the applicability of adiabatic models to FR I jets and identify the regions
where distributed particle acceleration is necessary
