When fluid phase behavior over a large region of states is considered, excess function models are no longer appropriate. They are designed to address mixing effects at constant density or pressure. Effects of varying density are most conveniently treated in terms of an equation of state. The thermodynamic relations for computing fluid phase behavior from an equation of state in combination with ideal gas properties are well established; cf. Section 2.1. Although they are more demanding computationally than excess function models, there are now many well-tested computer codes available that allow the computation of fluid phase behavior from an equation of state. Basically, this approach is free from any of the restrictions associated with the use of excess function models. In principle, an equation of state model is generally applicable, including simple and complicated molecules, and, in particular, mixtures of small and large molecules, as in polymer solutions. In practice, different equation of state models are used for different applications and no general model suitable for all applications has yet emerged. The generality of the equation of state approach requires full generality of the potential energy model. A formulation in terms of contact energies, adequate for excess function models, is unsufficient. Rather, the potential energy will in principle depend on the distances between the molecular centers, on the orientations of the molecules, and, in the most general case, also on their internal coordinates. In this book we shall concentrate on equation of state models for systems composed of small molecules, such as those typically encountered in the gas industries.