  Experimental results on high transverse momentum (leading) hadron spectra in
nucleus-nucleus collisions in the range sqrt(s_NN) = 20 -- 200 GeV are reviewed
with an emphasis on the observed suppression compared to free space production
in proton-proton collisions at the corresponding center-of-mass energies. The
transverse-momentum and collision-energy (but seemingly not the in-medium path
length) dependence of the experimental suppression factors measured in central
collisions is consistent with the expectations of final-state non-Abelian
parton energy loss in a dense QCD medium.
