Objective: Formally, incompetence implies that a patient cannot meet the legal requirements for informed consent. Our aim was to review the scientific literature on the relationship between competence and insight in patients with psychiatric disorders, how competence and insight are connected in these patients and whether there are differences in competence and insight among patients with different disorders. Method: A search in PubMed/Medline was performed. Articles were assessed on relevance criteria by two independent reviewers. Study design, population, variables, and outcomes were extracted. Results: Seven articles were included on studies of psychiatric inpatients and outpatients and of psychotic and non-psychotic patients. All studies used the MacArthur Competence Assessment Tool (MacCAT). All studies but one found a strong correlation between poor insight and incompetence. Psychotic patients with poor insight are very likely to be incompetent, and psychotic patients with adequate insight are generally competent. One well-executed study showed that in non-psychotic disorders, however, another relationship emerges; competence and insight do not completely overlap in these patients. Conclusion: Most incompetent psychotic patients have poor insight, but non-psychotic patients with adequate insight were incompetent in a substantial number of cases. In sum: non-psychotic patients with adequate insight can be incompetent. © 2011 John Wiley & Sons A/S.