In designing electrochemical or photoelectrical water splitting systems, the choice of electrolyte and possible membrane separators has a major interplay with the catalytic activity and overall performance of the water splitting system. To facilitate the optimal pH in both the catholyte and anolyte, a bipolar membrane has been introduced recently. Bipolar membranes dissociate water into H+ and OH-, where the H+ is supplied to the cathode and OH- supplied to the anode, and thereby balancing the consumption of these ions in the hydrogen and oxygen evolution, respectively. As a result, the pH in the catholyte and anolyte can be chosen and maintained independently. This chapter discusses the use of both monopolar membranes and bipolar membranes in (photo)electrochemical water splitting systems and provides an overview of the research in this field, including the principle of water dissociation, fabrication of bipolar membranes and use of bipolar membranes in other applications.