A Flow-Battery System is an Electrical Energy Storage approach that was originally conceived by NASA during the energy crises of the 1970s. A flow battery utilizes reversible redox couples on two electrodes to store chemical energy. However, instead of storing the electrochemical reactants within the electrode, as in a conventional battery, the reactants are dissolved in electrolytic solutions and stored in tanks external to the flow battery stack. Flow batteries are emerging as a potential electricity storage technology to support a more efficient, reliable, and cleaner electrical energy market. Some of the promising applications of flow batteries are related to load management of large-scale electricity supply to the grid (e.g., peak shaving, power quality, spinning reserves). Flow battery technology can also offer solutions to issues associated with the integration of intermittent renewable energy resources (e.g., wind, solar) with the power grid by making these power resources more stable, dependable, and dispachable. The objective of this paper is to provide an overview, status, and challenges of the flow-battery technology with an emphasis on vanadium redox-based system, which will also include an examination of recent results demonstrated by the United Technologies Research Center. Progress in the area of membranes for vanadium redox flow battery applications will be highlighted including a discussion of the important membrane improvements that would enable significantly higher power density which is needed to accelerate the commercialization of flow-battery technology.