Salinity-gradient energy or Blue Energy is a promising renewable energy source for the future. Estimates from literature predicted coverage of over 80% of the current global electricity demand when applied in all river mouths. From thermodynamic calculations it can be derived that each m3 of river water can yield 1.4 MJ when mixed with the same amount of sea water. Two techniques are available to convert Blue Energy into electricity: pressureretarded osmosis and reverse electrodialysis. For further research in this thesis, the latter was selected. From a review we concluded it has better prospects for river mouths regarding power density, energy recovery, fouling behavior and process economy. Until now, it has been investigated generally with a focus on obtained power, without taking care of the energy recovery. In this thesis, we emphasized the aspect of energy recovery. In our opinion, this is the most critical factor to success. We were the first to obtain a significant energy recovery of over 80%. Another important issue of this thesis is how the system will behave in practice, i.e., when it is applied to feed waters with different chemical compositions and biological activity. We investigated different operations and designs in relation to biofouling. Regarding the operations, a periodically applied feed water reversal hampers the biofouling significantly (extending the operational period with a factor 4). Regarding the design, a proof of principle was given of a newly designed spacer-free stack. This design leads to better performances (power density and energy recovery) and is less sensitive to biofouling. Based on this work, we defined requirements for membrane development and stack design (in relation to pre-treatment and friction losses). We also re-examined the economic feasibility and the global and national prospects of Blue Energy.