The classification of functional psychoses is still a controversial issue, as are also diagnoses in psychiatry. The predictive validity of the diagnosis is of crucial importance. Diagnostic systems are discussed. The author presents the Scandinavian concept of reactive psychoses, schizophreniform psychoses and schizophrenia, and demonstrates from his own material on paranoid psychoses the predictive value of these concepts, with a percentage recovery of 81 61 and 23 % after long-term follow-up. The concepts are discussed in relation to ICD-9 and DSM-III. The concepts of paranoid disorders, affective disorders and borderline conditions are mentioned. The paper also introduces other papers to be presented in this volume.