Ionomers, that is polymers containing a low concentration of charged units along the chain, have been the subject of increasing interest during the past twenty years. The presence of ionic groups in the poly­ mer changes some of its properties dramatically. Increases in the modu­ lus and the viscosity of several orders of magnitude have been observed, and changes in the glass transition of hundreds of degrees are possible. In addition, diffusion coefficients can be modified drastically. These changes are due primarily to the presence of reversible ionic cross­ links in these materials. Because of the low dielectric constant of most organic polymers, the ions or ionic dipoles tend to aggregate ; this aggregation process, however, is limited, because the ionic groups are covalently bonded to the organic chain. Host of the fundamental research done on these materials has been devoted to a determination of the extent of association, the structure of the aggregates, the limi­ ting factors, and the correlations between molecular and supermolecular structure and the resul ting properties.