We will now apply our knowledge of modeling to a system of biological importance. This model-building exercise is a reasonably detailed examination of the forces that exist between ions and a solvent. The treatment of these forces will lead to an understanding, in molecular terms, of the deviations from the ideal, characterized empirically by the activity coefficient. Initially, a continuum model that neglects any knowledge of the molecular structure of water will be proposed. The empirically inspired corrections to the model will point the way toward a more detailed examination of ion–solvent interactions. Initially, the behavior of ion–solvent interactions will be described in terms of simple electrostatics, but this will turn out to be a crude approximation. As further detail about the solvent and its interaction with ions is sought, however, it will become apparent that the structure of water has important effects in modifying the electrostatic environment. In considering the dielectric constant and the Kirkwood equations, the effects of water structure on the electrostatic forces are described in some detail, an exercise that is important when considering the more complex relationships in cellular systems.