At the start of this project, the authors were sceptical of current psychiatric nosology, believing it possible to produce a biological basis for a classification of mental disorders which would be more satisfactory than that embodied in DSM-IV or ICD 10. However, when our analysis was complete, we found ourselves to be in agreement with many aspects of the current scheme. On the whole, there are good biological reasons for upholding the distinctions between the schizophrenic and affective psychoses, between neurotic (reactive), and psychotic (endogenous) forms of depression, and between three categories of personality disorder.