Common to all these descriptions is a series expansion of ln γ ± in the molality or, more precisely, ionic strength framework starting from the limit of infinite dilution. In most cases the dilute limit is described by a Debye-Hückel term. Each series expansion takes into account individual ion-ion interactions over the full volume in the configuration integral. The behavior at high molalities is in general derived by assuming ion specific (hydrated) ion radii which are fitted to reproduce the experimental activity coefficients. The ability of ions to form different types of ion pairs or, more general, ion complexes makes an evaluation at high concentration difficult.