As discussed in chapter 1, the main application of characterization methods presented in Chapters 2-4 is to provide basic data for estimation of various thermophysical properties of petroleum fractions and crude oils. These properties are calculated through thermodynamic relations. Although some of these correlations are empirically developed, most of them are based on sound thermodynamic and physical principles. The most important thermodynamic relation is pressure-volume-temperature (PVT) relation. Mathematical PVT relations are known as equations of state. Once the PVT relation for a fluid is known various physical and thermodynamic properties can be obtained through appropriate relations that will be discussed in Chapter 6. In this chapter we review principles and theory of property estimation methods and equations of states that are needed to calculate various thermophysical properties.