Grid-scale energy storage systems are of interest as the world increases reliance on renewable energy sources. Redox flow batteries are a type of grid-scale energy storage technology that shows exceptional promise for accommodating the dynamic output of wind and solar power sources. The research community employs dozens of diagnostic techniques to investigate nearly all facets of these devices. Material properties, operational losses, transport, and integrated system properties are studied through the lens of electrochemical, physical, and chemical phenomena that ultimately dictate cost by influencing efficiency, durability, power, and capacity. These diagnostic techniques can, if applied correctly, elucidate not only the types of losses in redox flow batteries, but also tie those losses to fundamental driving forces in such systems so that next generation systems and models can be designed. This review details various diagnostic techniques used in flow battery analysis. The benefits, unique insights, and limitations of these techniques are discussed. Recommendations are also made to assist researchers in identifying the diagnostics that can advance their particular investigations. The review concludes with a summary of opportunities for new diagnostics that are needed to enable solution of persistent issues in redox flow battery research and development. (C) The Author(s) 2018. Published by ECS.