Objective - To discuss some theoretical and practical problems related to classification in the medical and psychiatric fields, and to present the general structure of the Tenth Revision of the International Classification of Diseases (ICD-10), officially adopted as from January 1993. Results - This paper discusses the epistemological basis of classification, the various classification strategies adopted in the medical and psychiatric fields and the main theoretical models employed in this area. The problems related to the reliability and validity of psychiatric diagnoses are then reviewed, and the different strategies to be used in order to improve them are presented. Finally the principles which have led the development of the ICD-10, and the field trials carried out in order to test the appropriatness of this system are discussed. Conclusions - In the psychiatric field classification is essential in order to make possible an ordered systematization of new knowledge and to facilitate the communication between health workers and users of services. The ICD-10 represents the biggest effort ever carried out at an international level to build a reliable classification incorporating different psychiatric traditions.

Objective - To discuss, after a description of the main models of classification of affective disorders, the main features of the ICD-10 classification of these disorders, the main differences between ICD-10 and ICD-9 and a comparison with DSM-III-R classification. Results - The analysis of the classification of these disorders as reported in ICD-10 shows that their nosographic sistematization is a complex task. In any case ICD-10 represents a significant improvement compared to ICD-9 and has included many of the guidelines of DSM-III-R. Conclusions - ICD-10 proposes a syntesis between different schools and traditions taking into account the results emerged by research in this field over the last twenty years.

Scope - To discuss, after a description of the main attempts of classification of schizophrenic disorders made from the last century up to now, the main features of the ICD-10 classification of schizophrenia and related dis-orders. The main features of this chapter of the ICD-10 are then compared to the ICD-9 and DSM-III-R classifications. Results - The ICD-10 diagnostic criteria for schizophrenia require a duration of only 1 month (compared to the 6 month duration criterion of the DSM-III-R); positive symptoms of psychosis, in particular Schneiderian symptoms, are given more prominence than they are in the DSM-III-R. Moreover, it is foreseen the possibility of a favourable outcome of the disorder; this has been based on the results of several follow-up studies, including some major WHO multicentric studies showing a positive outcome in substantial proportion of patients. Conclusions - On the whole, ICD-10 broadens the diagnostic concept of schizophrenia in comparison to other diagnostic systems, whereas the emphasis placed on Schneiderian first-rank symptoms tends to overweight this tendency and makes the criteria for inclusion more narrow.