Current psychiatric classification adopts a disorder-focused diagnostic approach, as exemplified within ICD-11 and DSM-V. Although this approach has improved reliability of categorization, its validity and utility has been questioned. Limitations include high comorbidity between supposedly distinct disorders; heterogeneity within diagnoses; limited treatment efficacy; and similarities across disorders in aetiology, latent symptom structure, and underlying biology. There is also evidence of transdiagnostic cognitive-behavioural processes. An alternative approach is therefore to focus on fundamental underlying mechanisms of psychopathology rather than observed symptom clusters. (...) This article considers the possible benefits, hurdles, and steps towards implementation of this transdiagnostic mechanistic approach, using the example of repetitive negative thought. (shrink)