In 8 studies, the authors investigated negative self-thinking as a mental habit. Mental content (negative self-thoughts) was distinguished from mental process (negative self-thinking habit). The negative self-thinking habit was assessed with a metacognitive instrument (Habit Index of Negative Thinking; HINT) measuring whether negative self-thoughts occur often, are unintended, are initiated without awareness, are difficult to control, and are self-descriptive. Controlling for negative cognitive content, the authors found that negative self-thinking habit was distinct from rumination and mindfulness, predicted explicit as well as implicit low self-esteem (name letter effect), attenuated a positivity bias in the processing of self-relevant stimuli, and predicted anxiety and depressive symptoms 9 months later. The results support the assumption that metacognitive reflection on negative self-thinking as mental habit may play an important role in self-evaluative processes.