Energy storage related to transient energy sources, such as solar and wind, and their integration with the energy distribution grid is getting increased attention. The characteristics required for this type of storage are quite different from those for energy storage in portable devices. Size and weight are not so important. Instead, matters such as power, cost, calendar life, cycle life, and safety are critical. A new family of hexacyanoferrate materials with the same type of open framework crystal structure as Prussian Blue has been recently developed with characteristics ideally suited for this type of application. Several monovalent cations can be rapidly and reversibly inserted into these materials, with very little crystallographic distortion. This can result in high rates and long cycle lives. A new type of composite negative electrode materials has also been developed that has the rapid kinetics typical of carbon electrodes, with a potential that varies little with the state of charge. The result is the development of a new battery system, the ferrocyanide / stabilized carbon, MHCF-SC, system. It is the purpose of this paper to review these developments. (C) The Author(s) 2017. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.