Reverse electrodialysis (RED) is an emerging membrane based technology that captures electricity from controlled mixing of two water streams of different salinities. To date, great advancements have been achieved on the development of RED components (e.g., membranes and spacers), optimization of operational conditions, and development of hybrid processes. This review presents an overview on the current achievements in RED membranes and spacers. Meanwhile, the critical operation conditions and their interconnected relationships are highlighted. Moreover, several innovative hybrid systems that show strong synergistic effects are highlighted. The latest development of nano-/micro-fluidic RED and pilot scale tests are also summarized.