Spent nuclear fuel, the used fuel removed from nuclear reactors, is one of the most hazardous substances created by humans. Commercial spent fuel is stored at reactor sites; about 74 percent of it is stored in pools of water, and 26 percent has been transferred to dry storage casks. The United States has no permanent disposal site for the nearly 70, 000 metric tons of spent fuel currently stored in 33 states. This book examines the amount of spent fuel expected to accumulate before it can be moved from commercial nuclear reactor sites; the key risks posed by stored spent nuclear fuel and actions to help mitigate these risks; and the key benefits and challenges of moving spent nuclear fuel out of wet storage and ultimately away from commercial nuclear reactors.