Publisher Summary The combination of hydrophilic and hydrophobic groups in the same polymer molecule of polymeric fluorinated ionomers results in unique properties and morphologies. This process has attracted the attention of industry, researchers, and theoreticians. Many questions regarding the inner workings of this material remain still unanswered. Both partially fluorinated and perfluorinated polymers, containing sufficient ionic groups to dominate the transport properties of the polymer, have been discussed. Ionic groups may include sulfonic and carboxylic groups as well as sulfonamides and sulfonimides. Due to their importance in the synthesis and fabrication of these ionomers, precursor polymers, containing sulfonyl fluoride or carboxylic ester groups are also discussed. It should be emphasized that these precursor polymers are not ionomers, and that they have properties which are quite different from those of the corresponding ionomers. The synthesis of a perfluorinated ionomer containing phosphonic acid groups has been discussed in detail. Perfluorinated ionomers containing sulfonyl imide functional groups have also received some attention. It remains to be seen whether the control of important polymerization parameters, such as molecular weight (MW) and its distribution, will allow the realization of the promise of inherently superior properties. Most fluorinated ionomers are sold as flat sheets and films, such as extruded or solution cast films, or as composite membranes containing fabric reinforcement, which is added to one or more layers of the ionomer. Extruded capillary tubing is also available. Smaller quantities are sold in the form of pellets for applications such as catalysts or for conversion to liquid compositions.