The article reviews research on repetitive thought (RT), which can have both constructive and unconstructive consequences. Unconstructive outcomes include depression, anxiety, and physical health difficulties, while constructive outcomes include recovery from traumatic events, adaptive planning, recovery from depression, and health-promoting behaviors. Key factors influencing RT's consequences include the valence of thought content, context, and level of construal (abstract vs. concrete). The control theory account is proposed as the best framework to explain RT's distinct consequences. RT is a common mental process involving repetitive thinking about self, concerns, and experiences. It includes constructs like depressive rumination, rumination, worry, perseverative cognition, cognitive and emotional processing, planning, problem-solving, mental simulation, counterfactual thinking, defensive pessimism, reflection, mind wandering, post-event rumination, positive rumination, and habitual negative self-thinking. These constructs are associated with both constructive and unconstructive outcomes. Research shows that RT is linked to vulnerability to depression, anxiety, and physical health issues. Cross-sectional studies indicate that RT is elevated in depressed individuals and associated with depressive symptoms. Prospective longitudinal studies find that RT predicts future depression, with depressive rumination mediating the effect of other risk factors. Experimental studies show that RT increases negative mood and cognition, particularly when participants are already in a dysphoric mood. The effects of RT are moderated by intrapersonal context, such as self-esteem and dysfunctional attitudes, and by the level of abstract versus concrete processing during RT. The review highlights that RT is a key factor in the onset and maintenance of depression and anxiety, with both constructive and unconstructive outcomes depending on context and thought content. The control theory account is proposed as the best framework to explain these findings.The article reviews research on repetitive thought (RT), which can have both constructive and unconstructive consequences. Unconstructive outcomes include depression, anxiety, and physical health difficulties, while constructive outcomes include recovery from traumatic events, adaptive planning, recovery from depression, and health-promoting behaviors. Key factors influencing RT's consequences include the valence of thought content, context, and level of construal (abstract vs. concrete). The control theory account is proposed as the best framework to explain RT's distinct consequences. RT is a common mental process involving repetitive thinking about self, concerns, and experiences. It includes constructs like depressive rumination, rumination, worry, perseverative cognition, cognitive and emotional processing, planning, problem-solving, mental simulation, counterfactual thinking, defensive pessimism, reflection, mind wandering, post-event rumination, positive rumination, and habitual negative self-thinking. These constructs are associated with both constructive and unconstructive outcomes. Research shows that RT is linked to vulnerability to depression, anxiety, and physical health issues. Cross-sectional studies indicate that RT is elevated in depressed individuals and associated with depressive symptoms. Prospective longitudinal studies find that RT predicts future depression, with depressive rumination mediating the effect of other risk factors. Experimental studies show that RT increases negative mood and cognition, particularly when participants are already in a dysphoric mood. The effects of RT are moderated by intrapersonal context, such as self-esteem and dysfunctional attitudes, and by the level of abstract versus concrete processing during RT. The review highlights that RT is a key factor in the onset and maintenance of depression and anxiety, with both constructive and unconstructive outcomes depending on context and thought content. The control theory account is proposed as the best framework to explain these findings.