The diagnosis of schizophrenia cannot be made based on the results of an ob­jective diagnostic test or laboratory measure, though we and others are working towards this. Instead, clinicians diagnose schizophrenia based on behaviour and psychopathology (including the symptoms described in the previous chapter). These require the subjective interpretation of clinicians, but they can be as­sessed reliably. The definitions of major mental illnesses used by clinicians are presented in the Diagnostic and Statistical Manual (DSM) of the American Psychiatric Association (in the United States) and the World Health Organization’s International Classification of Diseases (ICD) in other countries. These def­initions are updated from time to time to reflect gains in knowledge, or to reflect modern thinking on the similarities and differences between certain disorders. From one edition to the next, some diagnoses are revised, some are added, and some vanish altogether, only to be replaced or absorbed under other diagnoses. The diagnostic criteria for schizophrenia as defined by the most recent version of the DSM (DSM- 5) include the presence of two or more of the following symptoms: delusions, hallucinations, disorganized speech, disorganized or cata­tonic behaviour, and negative symptoms. At least one of the two must be delu­sions, hallucinations, or disorganized speech, while the second symptom type required for diagnosis could be any of the remaining four criteria. The require­ment of delusions, hallucinations, or disorganized speech maintains the resem­blance of the modern- day diagnosis to that first described by the clinician Emil Kraepelin over a century ago. Kraepelin’s discovery that schizophrenia is marked by a chronic and gradually worsening course is seen in modern- day criteria as well. A DSM-5 diagnosis of schizophrenia requires continuous signs of illness for at least 6 months, during which the individual must show at least 1 month of active symptoms (less if well treated). The diagnosis also requires social or work deterioration over a signifi­cant amount of time. Lastly, the diagnosis requires that the observed symptoms are not due to some other medical condition, including other psychiatric disorders such as bipolar disorder or major depressive disorder.