Each day researchers, entrepreneurs and many others across the United States are working to develop and deploy clean energy technologies that will enhance our security, reduce pollution and promote prosperity. Many new and emerging clean energy technologies, such as the components of wind turbines and electric vehicles, depend on materials with unique properties. The availability of a number of these materials is at risk due to their location, vulnerability to supply disruptions and lack of suitable substitutes. Ensuring reliable access to critical materials will help the United States lead in the new clean energy economy. This book examines the role of rare earth metals and other materials in the clean energy economy, based on data collected and researched by the U.S. Department of Energy.