Hard processes in nucleus-nucleus interactions at relativistic energies are reviewed with emphasis on recent PHENIX results from the first run of the Relativistic Heavy-Ion Collider at BNL. The observed suppression of moderately high $p_T$ hadrons ($p_T$ = 2 - 5 GeV/c) in $\sqrt{s_{NN}}$ = 130 GeV $Au+Au$ central collisions with respect to the scaled $pp$ data, is discussed in terms of conventional nuclear and ``quark-gluon-plasma'' effects. The meson and baryon composition at high $p_T$, as well as the implications for open charm of the measured single electron spectrum are also presented.