While approximately 20 years ago the design of separation processes required expensive and time consuming experiments, modern thermodynamic models nowadays allow a reliable prediction of the phase equilibrium behavior of multicomponent systems using binary data alone. If binary data are not available, group contribution methods can be applied to predict the real behavior. These methods have been developed for the prediction of vapor-liquid equilibria. The range of application and the reliability of these models were improved by modification of the model, definition of new main groups and introduction of temperature dependent parameters. Especially promising models arise on utilising g(E)-mixing rules in cubic equations of state. These models can be applied for the calculation and prediction of phase equilibria of strongly polar systems, including supercritical compounds. At the same time these models allow the calculation of densities, thermodynamic properties, etc.