  Coherence, multiple scattering and the interplay between soft and hard
processes are discussed. These physics phenomena are essential for
understanding the nuclear dependences of rapidity density and $p_T$ spectra in
high-energy heavy-ion collisions. The RHIC data have shown the onset of hard
processes and indications of high $p_T$ spectra suppression due to parton
energy loss. Within the pQCD parton model, the combination of azimuthal
anisotropy ($v_2$) and hadron spectra suppression at large $p_T$ can help one
to determine the initial gluon density in heavy-ion collisions at RHIC.
