Many important industrial applications, as well as insight into the phenomena of nature, rely crucially on knowledge about fluid phase behavior. In space and other high-temperature industries, as well as in combustion processes, the properties of gases manifesting various types of reaction, including dissociation and ionization, are required. In chemical and environmental science and technology, phase and reaction equilibria of multicomponent mixtures form the basis of understanding the phenomena and designing synthesis, separation, and purification processes. Biotechnological downstream processing relies on the distribution properties of biomolecules in different phases of aqueous and organic solutions. Even in standard mechanical engineering equipment technology, such as refrigerator design, lack of data for new environmentally friendly refrigerants has proved to be a severe obstacle to technological progress. In all these cases, and many others, fluid phase properties form the basis of modern technological processes and detailed and quantitative knowledge of their properties is the premise of innovation. Experimental studies alone, although indispensible in the field of fluid system science, cannot serve these needs. The project of studying the fluid phase behavior of a multicomponent system experimentally is hopeless in view of the large number of data that would be needed. Instead, molecular models, which can be evaluated on a computer and make use of the limited data available to predict the fluid phase behavior in the full range of interest, are needed. Due to the broad availability of high-speed computers, such models can be quite ambitious, including use of quantum-chemical and molecular simulation computer codes.