Equation-of-state (EOS) compositional reservoir simulation is an accurate and powerful means to model complicated phase and flow behavior involved in the displacement of oil and gas in porous media. The use of EOS simulation becomes even more effective when the process involves solvent injection for miscible or near-miscible displacement of crude oil in reservoirs. The current trend of combining the simulation of reservoirs and surface facilities increases the demand for the use of EOS compositional models. One key to proper use of compositional simulators is development of an EOS for describing the phase behavior of the fluids. This paper is a summary of experiences and developments in the use of EOS's for compositional reservoir simulation studies. The reader is referred to the recent monograph by Whitson and Brule(1) for a more complete treatment of the volumetric and phase behavior of petroleum fluids and the use of EOS's to model these fluids.