Monte Carlo Models: Quo Vadimus?

Abstract

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.