During the late 1990s, the Department of Energy (DOE) had difficulties with a lack of clear management authority and responsibility that contributed to security problems at the nation’s nuclear weapons laboratories and management problems with major projects. In response, Congress created the National Nuclear Security Administration (NNSA) as a separately organised agency within the DOE under the NNSA Act. The NNSA is responsible for managing nuclear weapons and non-proliferation-related national security activities in laboratories and other facilities, collectively known as the nuclear security enterprise. This book examines the problems that have arisen with the nuclear security enterprise, from projects’ cost and schedule overruns to inadequate oversight of safety and security at NNSA’s sites. With the NNSA proposing to spend tens of billions of dollars to modernise its facilities, it is important to ensure scarce resources are spent in an effective and efficient manner.