The Advanced Research Projects Agency-Energy (ARPA-E) was established in 2009 to address barriers to developing transformational science and technology solutions for the nation's energy and environmental challenges. As requested in the Energy Act of 2020, this report evaluates ARPA-E's operations and assesses the impacts of its programs to date. The report also explores the agency's future challenges and opportunities, recommending ways ARPA-E can build on its early successes to respond effectively to emerging energy needs. Key recommendations address funding and leadership stability, strategic planning and evaluation, risk and failure management, process innovation and experimentation, and transparency and public communication. This is the second National Academies review of ARPA-E, following the first report published in 2017.