Keeping U.S. nuclear weapons out of the hands of adversaries is essential for national security. On request of Congress, the National Nuclear Security Administration (NNSA) asked the National Academies for advice on improving security approaches. The Academies committee identified three major shortcomings: (1) the interactions and dependencies among security countermeasures, (2) the interactions between NNSA and other organizations responsible in part for preparing for or responding to an attack on NNSA facilities, and (3) the adequacy of attack scenarios used to design, update, and test the security systems to consider all possible attack scenarios.