A reliable knowledge of the phase equilibrium behavior of the system to be separated is a prerequisite for the synthesis, design and optimization of separation processes. This behavior can be calculated with the help of g(E) models and equations of state using binary data alone. When no experimental data are available, group contribution methods (ASOG, UNIFAC, modified UNIFAC, PSRK) can be successfully applied. For all these group contribution methods the Dortmund Data Bank (DDB) was used for fitting the required group interaction parameters. In this paper the present status of the different models will be shown and their advantages and disadvantages will be discussed. After a demonstration of typical results, the application for the synthesis of separation processes and an outlook on future developments will be given. Furthermore the importance of the Dortmund Data Bank and the integrated software packages for the development of reliable models with a large range of applicability and the application for the synthesis of separation processes will be demonstrated.