Central rapidity densities of charged particles at RHIC and LHC

Predictions on central rapidity densities of charged particles at energies of the relativistic heavy ion collider and the large hadron collider, for c entral collisions between the largest nuclei that will be available at thes e accelerators, are reviewed. Differences among the results of the existing models are discussed in relation with their underlying physical basis and with the possibilities to discriminate them.