The book guides the reader from the foundations of statisti-cal thermodynamics including the theory of intermolecularforces to modern computer-aided applications in chemical en-gineering and physical chemistry. The approach is new. Thefoundations of quantum and statistical mechanics are presen-ted in a simple way and their applications to the predictionof fluid phase behavior of real systems are demonstrated. Aparticular effort is made to introduce the reader to expli-cit formulations of intermolecular interaction models and toshow how these models influence the properties of fluid sy-stems. The established methods of statistical mechanics -computer simulation, perturbation theory, and numerical in-tegration - are discussed in a style appropriate for newcom-ers and are extensively applied. Numerous worked examplesillustrate how practical calculations should be carried out.