There exists a huge potential for the generation of energy from the mixing of saltwater and freshwater. The potential is 2.6 TW, which is more than the global electricity consumption (2.0 TW). Two membrane-based technologies exist to convert this potentially available energy into useful power: pressure-retarded osmosis (PRO) and reverse electrodialysis (RED). In PRO, water is transported through a semipermeable membrane from the less concentrated solution toward the concentrated salt solution to generate power. In RED, salt ions are transported from the concentrated salt solution through ion exchange membranes toward the less concentrated solution to extract the energy. Both technologies were developed in the 1970s and 1980s and both regained interest lately due to recent developments in membrane technology and the need for sustainable energy processes. This chapter describes the potential of salinity gradient energy of both processes in detail, and an overview of the relevant literature on both technologies is presented. Furthermore, it summarizes the recent developments, pilot testing, scale-up, and future expectations of both technologies.