  We present high transverse momentum ($p_T$) measurements made at the
Relativistic Heavy Ion Collider (RHIC) for Au+Au, d+Au, and p+p collisions at
$\sqrt{s_{NN}} = 200$ GeV, as well as for Au+Au collisions at $\sqrt{s_{NN}} =
62$ GeV. We observe a suppression in the yield of high $p_T$ hadrons measured
in central Au+Au collisions, relative to the yield in p+p collisions scaled by
the number of binary nucleon-nucleon collisions. This observation, together
with the absence of such a suppression in d+Au collisions, leads to the
conclusion that a dense medium is formed in central Au+Au collisions.
