As California, Hawaii, Vermont, and New York implement renewable portfolio standards of 50% and higher, long-duration electrical storage is one option that could play a key role in balancing intermittent renewable output. Long-duration storage serves the purpose of both supplementing intermittent renewable generation and replacing traditional fossil-fuel-powered peaker plants and, for the purpose of this talk, will be defined as a duration of 8 to 50 hours, or even longer. A recent NREL analysis of California’s future grid estimated that 15-28 GW of storage with an 8-h duration would be needed in order to maintain electricity prices at affordable levels if 50% of the state’s electricity was generated from solar PV. But these long-duration systems will be adopted only if they are safe, durable, and capable of storing energy at capital costs far lower than the 150 $/kWh (installed) that is targeted for approximately 5-hour systems. This presentation will help define a new long duration application area for electrochemical engineers and cover the following aspects of the concept: (i) What storage duration is required in a geographical region given various capacity values of renewable generators and degrees of renewable penetration? (ii) What system attributes are needed for long-duration storage? What are the appropriate approaches to balance capital cost, lifetime, energy density, and round-trip efficiency? (iii) What electrochemical storage technologies are most promising for long duration storage? Can lithium-ion serve these applications, or will new and unique battery and flow battery systems provide performance and cost advantages? (iv) How can various system design paradigms minimize balance of plant costs and the levelized cost of storage? (v) What is the broader set of technologies (other than batteries) that can address long duration storage for high renewables penetration (e.g., hydrogen storage, other electrical storage technologies, carbon capture and storage) of which electrochemical researchers should be aware?