Metal-free redox species for nonaqueous flow batteries emerged as flow-battery electrolytes in 2011. Over the past 10 years, they have developed from being unstable and unsuited to long-duration storage, to being designed with longevity and high-voltage applications in mind. The structural performance developments in the design of metal-free redox organic molecules are now such that these compounds have moved from being a topic of academic research interest into being viable molecules to take forward into new energy storage devices. This chapter explores the rapid development of such molecules that were once by-products of the lithium-ion battery industry to purposefully designed flow-battery electrolytes.