  Significant jet quenching in central $Au+Au$ collisions has been discovered
at RHIC. This paper provides theoretical arguments and lists experimental
evidence that the observed jet quenching at RHIC is due to parton energy loss
instead of hadron rescattering or absorption in a hadronic medium. These
include: (1) hadron formation time based on the uncertainty principle, (2)
$p_T$ dependence and (3) centrality dependence of the observed jet quenching,
(4) jet-like leading hadron correlations (5) high-$p_T$ azimuthal anisotropy
and (6) experimental data from $Pb+Pb$ collisions at SPS and $e+A$ collisions.
Direct measurements of the parton energy loss in the direction of a triggered
high-$p_T$ hadron and the medium modified fragmentation function on the
back-side are proposed to further verify the partonic nature of the observed
jet quenching. The importance of jet quenching studies at lower energies at
RHIC is also discussed.
