We describe in this chapter various models commonly used for relating the activity to the composition of condensed phases for binary and multicomponent systems. Because the ideal solution model provides a reference frame against which real solutions are assessed, we will begin with another discussion of ideal entropy of mixing. We will also review excess properties and the activity coefficients pertaining to the various composition scales before discussing solvent properties, such as osmotic pressure or changes in partial pressure and freezing point, which mainly depend on solute concentration. A few nonideal solution models of general interest for multicomponent systems, like the regular and subregular models, will then be examined before discussing specific cases like multisite and reciprocal solutions. We will finish with a description of the specific problems raised in electrolytes by ionic dissociation and Coulombic interactions and show how they are accounted for by the Debye-Huckel and Pitzer models.