This chapter provides a brief overview of some of the most important thermodynamic relations that may appear in the book. It also serves to fix notation and nomenclature. Special attention is given to the thermodynamic potential appropriate to each set of possible thermodynamic variables. For generality, the relations are referred to multicomponent fluid systems (mixtures) rather than to pure systems. Otherwise, the treatment is restricted to simple classical fluids and does not include magnetic or electric properties, external forces, surface properties, quantum effects, etc.