How mental disorder relates to criminal insanity is a contested matter. Norway has a tradition of using a ‘medical model’ for the definition of criminal insanity that is unique in an international perspective. According to this model, insanity is determined only in relation to a medical criterion, so that all that is required is the presence of a qualifying mental disorder. Criminal insanity is, under the current rule, equated with psychosis, although this rule has recently been subject to a law reform. This article explains and discusses this medical model by gathering together legal, forensic, and clinical empirical perspectives on the legal meaning and relevance of psychosis. The article will provide an explanation of the background of the medical model in Norwegian law, and the justifications for tying criminal insanity to psychosis. It will also explain how criminal insanity is operationalised in forensic practice, and discuss the legal conceptualisation of psychosis from a medical perspective. A main conclusion is that the legal meaning of psychosis is unclear, and the authors describe several challenges in legal and forensic practice. The authors emphasise the need for further knowledge development in the intersection between law and medicine.