Fossil fuel is scanty and negatively affects the earth by inducing climate change. As a result, alternative and renewable energy (RE) sources are being investigated to replace fossil fuel. Wind and solar energy are leaders in the RE market, but huge and accessible other energy sources such as salinity gradient power (SGP) exist and need to be collected to contribute to the global energy demand. SGP is mainly extracted with reverse electrodialysis (RED) and pressure retarded osmosis (PRO) systems. The applicability of RED-SGP in a natural environment is the current challenge due to the low power density and the high Levelized cost of energy (LCOE) of the process. The properties of the ion-exchange membranes (IEMs), the spacers, the feed solutions characteristics and salinity, and the electrodes are among the most important parameters that control the performance of RED-SGP processes. New highly selective, conductive, and cost-effective membranes are required to improve RED performance and reduce the produced energy cost. Alternatives feed solutions such as wastewaters and brine from desalination plants as well as hybrid RED such as RED combined to electrodialysis (ED), reverse osmosis (RO), or membrane distillation (MD) will result in cost-effective water treatment and SGP extraction.