The book guides the reader from the foundations of statistical thermodynamics including the theory of intermolecular forces to modern computer-aided applications in chemical engineering and physical chemistry. The approach is new. The foundations of quantum and statistical mechanics are presented in a simple way and their applications to the prediction of fluid phase behavior of real systems are demonstrated. A particular effort is made to introduce the reader to explicit formulations of intermolecular interaction models and to show how these models influence the properties of fluid systems. The established methods of statistical mechanics computer simulation, perturbation theory, and numerical integration - are discussed in a style appropriate for newcomers and are extensively applied. Numerous worked examples illustrate how practical calculations should be carried out.