Ionomers, i.e., predominantly hydrophobic polymers containing a modest amount of bonded acid or salt groups, have long been used in applications as diverse as packaging film, resilient covers for golf balls, synthetic drilling muds and proton exchange membranes. The nanophase microstructure of associated salt groups in ionomers produces unique physical, mechanical and transport properties that are advantageous in these and other industrial applications. In the 1980's and 90's, several research groups demonstrated that ionomers also may have practical use as minor additives in polymer systems, e.g., as compatibilizing agents in polymer blends. In this talk, we will review some of the properties of ionomers that are favorable for the applications mentioned above, and we will also discuss some new applications of ionomers – specifically, as reactive additives in composite materials, compatibilizers in polymer-clay nanocomposites and shape memory materials. We will focus our discussion on two types of ionomers: lightly sulfonated polystyrene and sulfonated EPDM.