  We summarize HST observations of the M87 jet, concentrating on polarimetry
and spectral index maps, and compare its optical and radio structures. The
evidence now supports a stratified model for the structure of the jet, whereby
high-energy, optical synchrotron emitting particles occupy physically different
regions of the jet, closer to the jet axis, with different magnetic field
configurations. It is in these regions where the shocks that produce the knots
in the inner jet appear to originate. Knot regions have optical spectra which
are much flatter than average for the jet, with the flattest-spectrum regions
coinciding with flux maxima of knots. These same regions are preceded by
regions where perpendicular magnetic fields are seen. Thus not only do we see
all the necessary ingredients for {\it in situ} particle acceleration in the
knots, but there is now fairly direct evidence for it as well. By tracking the
changes in radio-optical and optical spectral index in the knot regions, we can
comment on acceleration and cooling timescales in each knot.
