from the HydroWIRES program at WPTO, for sponsoring the project and providing valuable feedbacks.We also acknowledge the contributions of our partners on this project: Jeremy Twitchell of Pacific Northwest National Laboratory, who have assisted in developing this work with principles of energy storage dual-use in transmission services and market participation identified in a separated project report.Additionally, we acknowledge and appreciate the feedback and insightful comments provided throughout this project by many others, including David Kates of Nevada Hydro, Ziad Alaywan of ZGlobal Inc., Kun Zhu of the Midcontinent Independent System Operator (MISO), and Parag Mitra of the Electric Power Research Institute. We are deeply grateful to these individuals for lending their time and expertise to help the team better understand the impediments to dual-use storage and think through the potential implications of different approaches. The conclusions presented in this report are solely the responsibility of the authors. HydroWIRESIn April 2019, WPTO launched the HydroWIRES Initiative 1 to understand, enable, and improve hydropower and pumped storage hydropower's (PSH's) contributions to reliability, resilience, and integration in the rapidly evolving U.S. electricity system. The unique characteristics of hydropower, including PSH, make it well suited to provide a range of storage, generation flexibility, and other grid services to support the cost-effective integration of variable renewable resources.The U.S. electricity system is rapidly evolving, bringing both opportunities and challenges for the hydropower sector. While increasing deployment of variable renewables such as wind and solar have enabled low-cost, clean energy in many U.S. regions, it has also created a need for resources that can store energy or quickly change their operations to ensure a reliable and resilient grid. Hydropower (including PSH) is not only a supplier of bulk, low-cost, renewable energy but also a source of large-scale flexibility and a force multiplier for other renewable power generation sources. Realizing this potential requires innovation in several areas: understanding value drivers for hydropower under evolving system conditions, describing flexible capabilities and associated tradeoffs associated with hydropower meeting system needs, optimizing hydropower operations and planning, and developing innovative technologies that enable hydropower to operate more flexibly.