The Nuclear Regulatory Commission (NRC) relies on its staff's professional judgement in implementing its processes for overseeing the safety of U.S. commercial nuclear power reactors. In implementing this oversight, NRC allocates specific roles and responsibilities to resident inspectors assigned to each plant, regional officials at one of four regional offices responsible for most oversight activities, headquarters officials, and the nuclear power industry. This book examines how NRC implements its processes for overseeing the safety of commercial nuclear power reactors; the extent to which NRC consistently identifies and resolves findings through these processes; and NRC's methods for developing lessons learned to improve its oversight and challenges, if any, NRC faces in doing so. The authors also discuss the inspector general's assessment of the most serious management and performance challenges facing the NRC.