Publisher Summary This chapter covers high performance membranes, new and improved membranes introduced by the major manufacturers. It describes some commercially available products to help the reader in selecting a suitable membrane for a given application. It will also explain in detail the function of each membrane type. The manufacturers are reluctant to reveal the exact composition of their products, particularly of the more recently introduced ones. Nevertheless, a description of the older types will help the reader to understand the concepts involved. Web sites of manufacturers, provide data on the available membranes and resins. Unreinforced perfluorinated sulfonic acid films are available in a variety of thicknesses and are typically made of either 1000 or 1100 EW polymer. For most industrial electrolysis applications, fabric reinforcement is used to improve the mechanical strength, particularly the resistance to tear propagation. Sulfonic/Carboxylic membranes are used exclusively for the electrolysis of sodium chloride. They represent by far the largest volume and value of all fluorinated ionomer products. New membrane types are being developed for chlor-alkali applications mostly aimed at lower operating voltage or higher allowable current density. This would result in lower electric power consumption or lower investment cost respectively.