Repetitive thinking about negative experience, such as worry and rumination, is increasingly recognized as a transdiagnostic process underlying various forms of psychopathology including anxiety and depression. Recent theoretical models have emphasized the role of impaired attentional control and the habitual nature of negative biases in the development and maintenance of pathological repetitive thought. In this introduction, we provide a brief overview of these theories and of how the articles in the special series provide experimental evidence concerning these basic mechanisms underlying rumination and worry, and their relation to clinical dysfunction. Together the research summarized in these articles instantiates these theoretical frameworks and provides convergent evidence confirming the value of adopting a transdiagnostic approach that focuses directly on fundamental mechanisms of psychopathology, instead of on diagnostic criteria.