Nearly everyone has had the experience of worrying about things that may go wrong in the future, or dwelling on things that have gone wrong in the past.  Many people are able to dismiss worry, rumination, and other forms of perseverative thought (PT) with relative ease.  However, people with generalized anxiety disorder, obsessive-compulsive disorder, depression, and other emotional disorders often report being unable to stop worrying, obsessing, or ruminating once they have started.  I will describe a series of studies that draw on findings from clinical psychology, cognitive psychology, and cognitive neuroscience, to evaluate the roles of emotion regulation, cognitive control, and internal and external attention in the regulation of worry and other forms of PT in clinical and healthy populations.  Together, these studies highlight potential mechanisms of PT and promising new targets for neurocognitive therapies designed to reduce PT and anxiety.