Rumination is repetitive thinking about personal and self-related concerns. Such rumination focused on symptoms and feelings has been implicated in the onset and maintenance of depression, through both experimental and longitudinal prospective studies, consistent with the Response Styles Theory. Rumination is also conceptualized within a control theory perspective, as an instrumental response to unresolved personal goals. Rumination acts to exacerbate existing mood states and elaborate preexisting cognition—in this way, it can act as a vulnerability factor for psychopathology. However, there is emerging cutting edge evidence that rumination can also have adaptive consequences, when it either focuses on positive information or involves a processing mode that is more concrete, focused on the specific details and mechanics of situations. Rumination is also being proposed as a strong candidate for a transdiagnostic process that contributes to multiple emotional disorders. New approaches to the treatment of rumination have recently been developed with preliminary encouraging data, although further large-scale trials are required. Key issues for research into rumination going forward include more detailed unpacking of the underlying cognitive and attentional mechanisms determining individual differences in rumination and examining the contribution of rumination across physical and mental health.