Hadron freeze-out conditions in high energy nuclear collisions

Systematic trend of the hadron freeze-out conditions from AGS/SPS to RHIC is discussed. The most interesting results from collisions at RHIC are that the system is indeed approaching net-baryon free and the transverse expansion is much stronger than that from collisions at AGS/SPS energies. In order to understand the trend of the collective velocity, an energy scan between $\sqrt{s_{NN}} = 20 - 200 GeV, is important. In addition, systematic studies on the anisotropy parameter$v_2$and the transverse momentum distributions of$\phi, \Omega,$and$J / \psi$ are necessary as they will help in determining whether the collectivity is developed at the partonic stage.