  The two major pillars of searches for the Quark Gluon Plasma have been:
J/$\Psi$ suppression, proposed in 1986, and observed at both SPS fixed target
energies and at RHIC; and, more recently, the suppression of $\pi^0$ with
$p_T\geq 3$ GeV/c by a factor $\sim 5$ in Au+Au central collisions, observed at
RHIC in 2001, which had been predicted in advance as a consequence of
Landau-Pomeranchuck-Migdal coherent (gluon) bremsstrahlung by the outgoing
hard-scattered partons traversing the medium. However, new effects were
discovered and the quality of the measurements greatly improved so that the
clarity of the original explanations has become obscured. For instance:
J/$\Psi$ suppression is the same at SpS and RHIC. Is it the QGP, comovers,
something else? QCD provides beautiful explanations of $\pi^0$ and direct
$\gamma$ measurements in p-p collisions but precision fits of the best theories
of $\pi^0$ suppression barely agree with the Au+Au data. Better data are needed
for $10< p_T <20$ GeV/c, systematic errors are needed in theory calculations,
the values of parameters of the medium such as $\mean{\hat{q}}$ derived from
precision fits are the subject of controversy. ... These and other issues will
be discussed with a view to identify which conclusions are firm and where
further progress towards real understanding is required.
