Redox flow batteries are well suited to provide modular and scalable energy storage systems for a wide range of energy storage applications. In this paper, we review the development of redox-flow-battery technology including recent advances in new redox active materials, cell designs, and systems, all from the perspective of engineers interested in applying this technology. We discuss cost, performance, and reliability metrics that are critical for deployment of large flow-battery systems. The technology, while relatively young, has the potential for significant improvement through reduced materials costs, improved energy efficiency, and significant reduction in the overall system costs.