Ionomers are polymers with low dielectric constants, containing a relatively small number of ionic groups, and they usually show unique physical properties, for example, two glass‐transition temperatures, small‐angle x‐ray scattering peaks, unusual transport properties, and high melt viscosity. Since the 1950s, the ionic interaction has been known as a very important factor in modifying polymer properties, and thus has received considerable attention from industrial as well as academic arenas. This overview begins with a brief discussion on three models of ionomers, ie, core–shell model, hard‐sphere model, and Eisenberg–Hird–Moore model, and then moves on to glass transitions. Styrene‐based ionomers, presently the best‐understood family of ionomers in terms of structure–morphology–property relationships, are described. There is a discussion of crystalline ionomers, block ionomers, and end‐capped ionomer systems. Three different types of plasticization using polar, nonpolar, and amphiphilic plasticizers are discussed. Some major applications of ionomers are selectively demonstrated very briefly.