Differences in the thermodynamic treatment of electrolyte and nonelectrolyte solutions are highlighted. The relations between expressions for concentrations are stated and the need to consider the presence of ions is emphasized. Examples are presented to show that the activity coefficient of a particular ion depends on the nature of the other ions in the solution. An overview is given of the state of development - and shortcomings - of methods to,measure the activity coefficient of individual ions. Pitfalls in the use of polynomials to evaluate the mean ionic activity from measurements of the activity of the solvent are emphasized and solutions proposed. The interpretation of the mean ionic activity coefficient of Weak electrolytes is discussed. The relation between the excess Gibbs energy and the activity coefficients of the species in solutions is reviewed, and the relation between the McMillan-Mayer and Lewis-Randall frameworks is commented. An abstract of the basic equations for electrolyte solution, thermodynamics is appended.