Semipermeable membranes are widely used in gas separation, water purification, electrodialysis, fuel cells, and biomedical applications. Membranes are typically comprised of a thin polymeric material that permits selective transport of one or more chemical species. Membranes for fuel cells and other electrochemical applications have been extensively studied,^1–9  but the requirements for solar fuels devices demand the development of membranes with different properties than those of membranes designed for other applications.^10,11  Traditionally, work on solar fuels devices has focused on individual device components, such as optimization of electrodes for the oxygen evolution and hydrogen evolution reactions. However, the membrane performance is inextricably linked to that of the other components. With JCAP's focus on device and systems level considerations, membrane performance and design emerged as central design criteria that drive many other considerations, such as relevant electrolytes and component geometries. In the most basic solar fuels device architecture, the membrane is immersed in aqueous electrolyte between two planar electrodes, in common with many electrochemical device designs. To ensure that current can flow in the device and maintain overall electroneutrality, the membrane must permit the transport of electrolyte ions. However, to maximize device efficiency, the membrane must also limit the transport of oxidation and reduction products from one electrode to the other. The materials comprising typical membranes impose certain limitations on suitable electrolyte environments (i.e., pH), and therefore conditions for benchmarking and HTE discovery. These considerations are discussed in the chapters by McCrory (Chapter 5) and Gregoire (Chapter 9). The chapters by Weber (Chapter 13) and Xiang (Chapter 11) describe computational modeling and evaluation of prototype electrochemical water splitting devices incorporating membrane separators, further defining their performance requirements. Membrane materials requirements were outlined by Berger et al.,¹⁰  and will be discussed in more detail below. The membrane, therefore, has focused the research efforts in many facets of JCAP.