The direct production of electrical energy by reverse electrodialysis is a promising technology. Energy can be extracted from the salinity gradient between two salt solutions at different salt concentrations. From the resulting difference in osmotic energy of both solutions an electrical voltage and current can be generated by implementing ion exchange membranes. This is indicated as salinity gradient power by reverse electrodialysis (SGP-RE). Moreover, hybrid systems can be introduced that simultaneously produce potable water, thereby adding significantly to the economics of the process. This chapter describes the basic concepts of the SGP-RE process while pointing to the use and optimization window of ion exchange membranes as well as the use of other types of membranes within the hybrid systems, also generating potable water. Also the potential of using (stand-alone) SGP-RE batteries for storing solar energy and wind energy (or other (fluctuating) sources of energy) is indicated.