  In relativistic heavy ion collisions, the $p_T$ distributions of hadrons
reflect the transverse motion generated during the collision and therefore the
collision dynamics. The moderate $p_T$ values measured to date at $\sqrt{s}
\approx $ 20 GeV/nucleon contain a mix of hard and soft processes. The
available data from the SPS on $\pi$, K, p, and $\Lambda$ are used to constrain
the magnitude of collective radial expansion. In $A+A$ collisions, the
hydrodynamic distributions determined from the measured collective expansion
are determined up to high $p_T$ and are consistent with the $\pi^{0}$ data. To
cleanly observe hard processes, measurements at $p_T \ge 4$ GeV/c will be
required. Pions produced in $p+W$ collisions clearly deviate from hydrodynamic
distributions at $p_T > 2$ GeV/c, underscoring the differences in the physics
between $p+A$ and $A+A$ collisions.
